Magnetic tape device and head tracking servo method

ABSTRACT

The magnetic tape device includes a magnetic tape; and a servo head, in which the servo head is a TMR head, the magnetic tape includes a servo pattern in the magnetic layer, a center line average surface roughness Ra measured regarding a surface of the magnetic layer is equal to or smaller than 2.0 nm, a logarithmic decrement acquired by a pendulum viscoelasticity test performed regarding the surface of the magnetic layer is equal to or smaller than 0.050, and a ratio (Sdc/Sac) of an average area Sdc of a magnetic cluster of the magnetic tape in a DC demagnetization state and an average area Sac of a magnetic cluster thereof in an AC demagnetization state measured with a magnetic force microscope is 0.80 to 1.30.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2017-029496 filed on Feb. 20, 2017. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape device and a headtracking servo method.

2. Description of the Related Art

Magnetic recording is used as a method of recording information in arecording medium. In the magnetic recording, information is recorded ona magnetic recording medium as a magnetized pattern. Informationrecorded on a magnetic recording medium is reproduced by reading amagnetic signal obtained from the magnetized pattern by a magnetic head.As a magnetic head used for such reproducing, various magnetic headshave been proposed (for example, see JP2004-185676A).

SUMMARY OF THE INVENTION

An increase in recording capacity (high capacity) of a magneticrecording medium is required in accordance with a great increase ininformation content in recent years. As means for realizing highcapacity, a technology of increasing a recording density of a magneticrecording medium is used. However, as the recording density increases, amagnetic signal (specifically, a leakage magnetic field) obtained from amagnetic layer tends to become weak. Accordingly, it is desired that ahigh-sensitivity magnetic head capable of reading a weak signal withexcellent sensitivity is used as a reproducing head. Regarding thesensitivity of the magnetic head, it is said that a magnetoresistive(MR) head using a magnetoresistance effect as an operating principle hasexcellent sensitivity, compared to an inductive head used in the relatedart.

As the MR head, an anisotropic magnetoresistive (AMR) head and a giantmagnetoresistive (GMR) head are known as disclosed in a paragraph 0003of JP2004-185676A. The GMR head is an MR head having excellentsensitivity than that of the AMR head. In addition, a tunnelmagnetoresistive (TMR) head disclosed in a paragraph 0004 and the likeof JP2004-185676A is an MR head having a high possibility of realizinghigher sensitivity.

Meanwhile, a recording and reproducing system of the magnetic recordingis broadly divided into a levitation type and a sliding type. A magneticrecording medium in which information is recorded by the magneticrecording is broadly divided into a magnetic disk and a magnetic tape.Hereinafter, a drive including a magnetic disk as a magnetic recordingmedium is referred to as a “magnetic disk device” and a drive includinga magnetic tape as a magnetic recording medium is referred to as a“magnetic tape device”.

The magnetic disk device is generally called a hard disk drive (HDD) anda levitation type recording and reproducing system is used. In themagnetic disk device, a shape of a surface of a magnetic head sliderfacing a magnetic disk and a head suspension assembly that supports themagnetic head slider are designed so that a predetermined intervalbetween a magnetic disk and a magnetic head can be maintained with airflow at the time of rotation of the magnetic disk. In such a magneticdisk device, information is recorded and reproduced in a state where themagnetic disk and the magnetic head do not come into contact with eachother. The recording and reproducing system described above is thelevitation type. On the other hand, a sliding type recording andreproducing system is used in the magnetic tape device. In the magnetictape device, a surface of a magnetic layer of a magnetic tape and amagnetic head come into contact with each other and slide on each other,at the time of the recording and reproducing information.

JP2004-185676A proposes usage of the TMR head as a reproducing head forreproducing information in the magnetic disk device. On the other hand,the usage of the TMR head as a reproducing head in the magnetic tapedevice is currently still in a stage where the future usage thereof isexpected, and the usage thereof is not yet practically realized.

However, in the magnetic tape, information is normally recorded on adata band of the magnetic tape. Accordingly, data tracks are formed inthe data band. As means for realizing high capacity of the magnetictape, a technology of disposing the larger amount of data tracks in awidth direction of the magnetic tape by narrowing the width of the datatrack to increase recording density is used. However, in a case wherethe width of the data track is narrowed and the recording and/orreproduction of information is performed by transporting the magnetictape in the magnetic tape device, it is difficult that a magnetic headproperly follows the data tracks in accordance with the position changeof the magnetic tape, and errors may easily occur at the time ofrecording and/or reproduction. Thus, as means for preventing occurrenceof such errors, a method of forming a servo pattern in the magneticlayer and performing head tracking servo has been recently proposed andpractically used. In a magnetic servo type head tracking servo amonghead tracking servos, a servo pattern is formed in a magnetic layer of amagnetic tape, and this servo pattern is read by a servo head to performhead tracking servo. The head tracking servo is to control a position ofa magnetic head in the magnetic tape device. The head tracking servo ismore specifically performed as follows.

First, a servo head reads a servo pattern to be formed in a magneticlayer (that is, reproduces a servo signal). A position of a magnetichead in a magnetic tape device is controlled in accordance with a valueobtained by reading the servo pattern. Accordingly, in a case oftransporting the magnetic tape in the magnetic tape device for recordingand/or reproducing information, it is possible to increase an accuracyof the magnetic head following the data track, even in a case where theposition of the magnetic tape is changed. For example, even in a casewhere the position of the magnetic tape is changed in the widthdirection with respect to the magnetic head, in a case of recordingand/or reproducing information by transporting the magnetic tape in themagnetic tape device, it is possible to control the position of themagnetic head of the magnetic tape in the width direction in themagnetic tape device, by performing the head tracking servo. By doingso, it is possible to properly record information in the magnetic tapeand/or properly reproduce information recorded on the magnetic tape inthe magnetic tape device.

The servo pattern is formed by magnetizing a specific position of themagnetic layer. A plurality of regions including a servo pattern(referred to as “servo bands”) are generally present in the magnetictape capable of performing the head tracking servo along a longitudinaldirection. A region interposed between two servo bands is referred to asa data band. The recording of information is performed on the data bandand a plurality of data tracks are formed in each data band along thelongitudinal direction. In order to realize high capacity of themagnetic tape, it is preferable that the larger number of the data bandswhich are regions where information is recorded are present in themagnetic layer. As means for that, a technology of increasing apercentage of the data bands occupying the magnetic layer by narrowingthe width of the servo band which is not a region in which informationis recorded is considered. In regards to this point, the inventors haveconsidered that, since a read track width of the servo pattern becomesnarrow, in a case where the width of the servo band becomes narrow, itis desired to use a magnetic head having high sensitivity as the servohead, in order to ensure reading accuracy of the servo pattern. As amagnetic head for this, the inventors focused on a TMR head which hasbeen proposed to be used as a reproducing head in the magnetic diskdevice in JP2004-185676A. As described above, the usage of the TMR headin the magnetic tape device is still in a stage where the future usethereof as a reproducing head for reproducing information is expected,and the usage of the TMR head as the servo head has not even proposedyet. However, the inventors have thought that, it is possible to dealwith realization of higher sensitivity of the future magnetic tape, in acase where the TMR head is used as the servo head in the magnetic tapedevice which performs the head tracking servo.

In addition, a signal-to-noise-ratio (SNR) at the time of reading theservo pattern tends to decrease in accordance with a decrease in readtrack width of the servo pattern. However, a decrease in SNR at the timeof reading the servo pattern causes a decrease in accuracy that themagnetic head follows the data track by the head tracking servo.

Therefore, an object of the invention is to provide a magnetic tapedevice in which a TMR head is mounted as a servo head and a servopattern written on a magnetic tape can be read at a high SNR.

As means for increasing the SNR at the time of reproducing informationrecorded on the magnetic tape, a method of increasing smoothness of asurface of a magnetic layer of a magnetic tape is used. This point isalso preferable for increasing the SNR in a case of reading a servopattern written in the magnetic tape. The inventors have made intensivestudies for further increasing the SNR in a case of reading a servopattern written in the magnetic tape, by using other methods, inaddition to the method of increasing smoothness of a surface of amagnetic layer of a magnetic tape.

Meanwhile, a magnetoresistance effect which is an operating principle ofthe MR head such as the TMR head is a phenomenon in which electricresistance changes depending on a change in magnetic field. The MR headdetects a change in leakage magnetic field generated from a magneticrecording medium as a change in resistance value (electric resistance)and reproduces information by converting the change in resistance valueinto a change in voltage. In a case where the TMR head is used as theservo head, the TMR head detects a change in leakage magnetic fieldgenerated from a magnetic layer in which the servo pattern is formed, asa change in resistance value (electric resistance) and reads the servopattern (reproduces a servo signal) by converting the change inresistance value into a change in voltage. It is said that a resistancevalue in the TMR head is generally high, as disclosed in a paragraph0007 of JP2004-185676A, but generation of a significant decrease inresistance value in the TMR head, while continuing the reproducing of aservo pattern with the TMR head, may cause a decrease in sensitivity ofthe TMR head, while continuing the head tracking servo. As a result, theaccuracy of head position controlling of the head tracking servo maydecrease, while continuing the head tracking servo.

During intensive studies for achieving the object described above, theinventors have found a phenomenon which was not known in the relatedart, in that, in a case of using the TMR head as a servo head in themagnetic tape device which performs the head tracking servo, asignificant decrease in resistance value (electric resistance) occurs inthe TMR head. A decrease in resistance value in the TMR head is adecrease in electric resistance measured by bringing an electricresistance measuring device into contact with a wiring connecting twoelectrodes configuring a tunnel magnetoresistance effect type elementincluded in the TMR head. The phenomenon in which this resistance valuesignificantly decreases is not observed in a case of using the TMR headin the magnetic disk device, nor in a case of using other MR heads suchas the GMR head in the magnetic disk device or the magnetic tape device.That is, occurrence of a significant decrease in resistance value in theTMR head in a case of using the TMR head was not even confirmed in therelated art. A difference in the recording and reproducing systembetween the magnetic disk device and the magnetic tape device,specifically, contact and non-contact between a magnetic recordingmedium and a magnetic head may be the reason why a significant decreasein resistance value in the TMR head occurred in the magnetic tape deviceis not observed in the magnetic disk device. In addition, the TMR headhas a special structure in which two electrodes are provided with aninsulating layer (tunnel barrier layer) interposed therebetween in adirection in which a magnetic tape is transported, which is not appliedto other MR heads which are currently practically used, and it isconsidered that this is the reason why a significant decrease inresistance value occurring in the TMR head is not observed in other MRheads. It is clear that, a significant decrease in resistance value inthe TMR head tends to more significantly occur in a magnetic tape devicein which a magnetic tape having high smoothness of a surface of amagnetic layer is mounted as the magnetic tape. With respect to this, asa result of more intensive studies after finding the phenomenondescribed above, the inventors have newly found that such a significantdecrease in resistance value can be prevented by using a magnetic tapedescribed below as the magnetic tape.

One aspect of the invention has been completed based on the findingdescribed above.

That is, according to one aspect of the invention, there is provided amagnetic tape device comprising: a magnetic tape; and a servo head, inwhich the servo head is a magnetic head (hereinafter, also referred toas a “TMR head”) including a tunnel magnetoresistance effect typeelement (hereinafter, also referred to as a “TMR element”) as a servopattern reading element, the magnetic tape includes a non-magneticsupport, and a magnetic layer including ferromagnetic powder and abinding agent on the non-magnetic support, the magnetic layer includes aservo pattern, a center line average surface roughness Ra measuredregarding a surface of the magnetic layer (hereinafter, also referred toas a “magnetic layer surface roughness Ra”) is equal to or smaller than2.0 nm, a logarithmic decrement acquired by a pendulum viscoelasticitytest performed regarding the surface of the magnetic layer (hereinafter,also simply referred to as a “logarithmic decrement”) is equal to orsmaller than 0.050, and a ratio of an average area Sdc of a magneticcluster of the magnetic tape in a DC demagnetization state and anaverage area Sac of a magnetic cluster of the magnetic tape in an ACdemagnetization state measured with a magnetic force microscope(Sdc/Sac; hereinafter, also referred to as a “magnetic cluster arearatio Sdc/Sac”) is 0.80 to 1.30.

According to another aspect of the invention, there is provided a headtracking servo method comprising: reading a servo pattern of a magneticlayer of a magnetic tape by a servo head in a magnetic tape device, inwhich the servo head is a magnetic head including a tunnelmagnetoresistance effect type element as a servo pattern readingelement, the magnetic tape includes a non-magnetic support, and amagnetic layer including ferromagnetic powder and a binding agent on thenon-magnetic support, the magnetic layer includes the servo pattern, acenter line average surface roughness Ra measured regarding a surface ofthe magnetic layer (magnetic layer surface roughness Ra) is equal to orsmaller than 2.0 nm, a logarithmic decrement acquired by a pendulumviscoelasticity test performed regarding the surface of the magneticlayer is equal to or smaller than 0.050, and a ratio of an average areaSdc of magnetic clusters of the magnetic tape in a DC demagnetizationstate and an average area Sac of magnetic clusters thereof in an ACdemagnetization state measured with a magnetic force microscope(Sdc/Sac) is 0.80 to 1.30.

One aspect of the magnetic tape device and the head tracking servomethod is as follows.

In one aspect, the logarithmic decrement is 0.010 to 0.050.

In one aspect, the center line average surface roughness Ra measuredregarding the surface of the magnetic layer is 1.2 nm to 2.0 nm.

In one aspect, the magnetic tape includes a non-magnetic layer includingnon-magnetic powder and a binding agent between the non-magnetic supportand the magnetic layer.

According to one aspect of the invention, it is possible to perform thereading at a high SNR, in a case of reading a servo pattern of themagnetic layer of the magnetic tape with the TMR head and preventoccurrence of a significant decrease in resistance value in the TMRhead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram of a measurement method of alogarithmic decrement.

FIG. 2 is an explanatory diagram of the measurement method of alogarithmic decrement.

FIG. 3 is an explanatory diagram of the measurement method of alogarithmic decrement.

FIG. 4 shows an example (step schematic view) of a specific aspect of amagnetic tape manufacturing step.

FIG. 5 shows an example of disposition of data bands and servo bands.

FIG. 6 shows a servo pattern disposition example of a linear-tape-open(LTO) Ultrium format tape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Tape Device

One aspect of the invention relates to a magnetic tape device including:a magnetic tape; and a servo head, in which the servo head is a magnetichead including a tunnel magnetoresistance effect type element as a servopattern reading element, the magnetic tape includes a non-magneticsupport, and a magnetic layer including ferromagnetic powder and abinding agent on the non-magnetic support, the magnetic layer includesthe servo pattern, a center line average surface roughness Ra measuredregarding a surface of the magnetic layer (magnetic layer surfaceroughness Ra) is equal to or smaller than 2.0 nm, a logarithmicdecrement acquired by a pendulum viscoelasticity test performedregarding the surface of the magnetic layer is equal to or smaller than0.050, and a ratio of an average area Sdc of magnetic clusters of themagnetic tape in a DC demagnetization state and an average area Sac ofmagnetic clusters thereof in an AC demagnetization state measured with amagnetic force microscope (Sdc/Sac) is 0.80 to 1.30.

The inventors have thought that the magnetic layer surface roughness Raand the magnetic cluster area ratio Sdc/Sac set to be in the rangesdescribed above contribute to the reading of a servo pattern written inthe magnetic layer of the magnetic tape in the magnetic tape device at ahigh SNR, and logarithmic decrement set to be in the range describedabove contributes to the prevention of a significant decrease inresistance value in the TMR head.

The magnetic layer surface roughness Ra equal to or smaller than 2.0 nmcan contribute to a decrease in spacing loss causing a decrease in SNR.In addition, the magnetic cluster area ratio Sdc/Sac of 0.80 to 1.30also contribute to improvement of the SNR. It is thought that magneticcluster area ratio Sdc/Sac is a value which may be an index for a stateof ferromagnetic powder present in the magnetic layer. It is surmisedthat, a state in which the magnetic cluster area ratio Sdc/Sac is 0.80to 1.30 is a state in which aggregation of particles of ferromagneticpowder is prevented in the magnetic layer, and such a state contributesto the reading of a servo pattern written in the magnetic layer at ahigh SNR.

The above description is a surmise of the inventors regarding thereading of a servo pattern written in the magnetic layer of the magnetictape at a high SNR, in the magnetic tape device. The inventors havethought regarding the usage of the TMR head by preventing the occurrenceof a significant decrease in resistance value, in the magnetic tape.

In the magnetic tape device, in a case of using a magnetic tape of therelated art, in a case of using a TMR head as a servo head forperforming head tracking servo at the time of recording and/orreproducing information, a phenomenon in which a resistance value(electric resistance) significantly decreases in the TMR head occurs.This phenomenon is a phenomenon that has been newly found by theinventors. The inventors have considered the reason for the occurrenceof such a phenomenon is as follows.

The TMR head is a magnetic head using a tunnel magnetoresistance effectand includes two electrodes with an insulating layer (tunnel barrierlayer) interposed therebetween. The tunnel barrier layer positionedbetween the two electrodes is an insulating layer, and thus, even in acase where a voltage is applied between the two electrodes, in general,a current does not flow or does not substantially flow between theelectrodes. However, a current (tunnel current) flows by a tunnel effectdepending on a direction of a magnetic field of a free layer affected bya leakage magnetic field from the magnetic tape, and a change in amountof a tunnel current flow is detected as a change in resistance value bythe tunnel magnetoresistance effect. By converting the change inresistance value into a change in voltage, a servo pattern formed in themagnetic tape can be read (a servo signal can be reproduced).

Examples of a structure of the MR head include a current-in-plane (CIP)structure and a current-perpendicular-to-plane (CPP) structure, and theTMR head is a magnetic head having a CPP structure. In the MR headhaving a CPP structure, a current flows in a direction perpendicular toa film surface of an MR element, that is, a direction in which themagnetic tape is transported, in a case of reading a servo patternformed in the magnetic tape. With respect to this, other MR heads, forexample, a spin valve type GMR head which is widely used in recent yearsamong the GMR heads has a CIP structure. In the MR head having a CIPstructure, a current flows in a direction in a film plane of an MRelement, that is, a direction perpendicular to a direction in which themagnetic tape is transported, in a case of reading a servo patternformed in the magnetic tape.

As described above, the TMR head has a special structure which is notapplied to other MR heads which are currently practically used.Accordingly, in a case where short circuit (bypass due to damage) occurseven at one portion between the two electrodes, the resistance valuesignificantly decreases. A significant decrease in resistance value in acase of the short circuit occurred even at one portion between the twoelectrodes as described above is a phenomenon which does not occur inother MR heads. In the magnetic disk device using a levitation typerecording and reproducing system, a magnetic disk and a magnetic head donot come into contact with each other, and thus, damage causing shortcircuit hardly occurs. On the other hand, in the magnetic tape deviceusing a sliding type recording and reproducing system, the magnetic tapeand the servo head come into contact with each other and slide on eachother, in a case of reading a servo pattern by the servo head.Accordingly, in a case where any measures are not prepared, the TMR headis damaged due to the sliding between the TMR head and the magnetictape, and thus, short circuit easily occurs. The inventors have assumedthat this is the reason why a decrease in resistance value of the TMRhead significantly occurs, in a case of using the TMR head as the servohead in the magnetic tape device. In addition, it is thought that, in acase where the smoothness of the surface of the magnetic layer of themagnetic tape increases, a contact area (so-called real contact area)between the surface of the magnetic layer and the servo head increases.It is thought that the servo head which is more easily damaged at thetime of sliding on the magnetic tape due to an increase in contact area,is a reason a decrease in resistance value in the TMR head which tendsto be significant, in the magnetic tape device in which the magnetictape having high smoothness of the surface of the magnetic layer ismounted.

With respect to this, as a result of intensive studies of the inventors,the inventors have newly found that it is possible to prevent aphenomenon in which a decrease in resistance value of the TMR headoccurs significantly, in a case of using the TMR head as a servo head inthe magnetic tape device, by using the magnetic tape in which alogarithmic decrement acquired by a pendulum viscoelasticity testperformed regarding the surface of the magnetic layer is equal to orsmaller than 0.050. This point will be further described below.

In the invention and the specification, the magnetic layer sidelogarithmic decrement is a value acquired by the following method.

FIGS. 1 to 3 are explanatory diagrams of a measurement method of thelogarithmic decrement. Hereinafter, the measurement method of thelogarithmic decrement will be described with reference to the drawings.However, the aspect shown in the drawing is merely an example and theinvention is not limited thereto.

A measurement sample 100 is cut out from the magnetic tape which is ameasurement target. The cut-out measurement sample 100 is placed on asubstrate 103 so that a measurement surface (surface of the magneticlayer) faces upwards, in a sample stage 101 in a pendulumviscoelasticity tester, and the measurement sample is fixed by fixingtapes 105 in a state where obvious wrinkles which can be visuallyconfirmed are not generated.

A pendulum-attached columnar cylinder edge 104 (diameter of 4 mm) havingmass of 13 g is loaded on the measurement surface of the measurementsample 100 so that a long axis direction of the cylinder edge becomesparallel to a longitudinal direction of the measurement sample 100. Anexample of a state in which the pendulum-attached columnar cylinder edge104 is loaded on the measurement surface of the measurement sample 100as described above (state seen from the top) is shown in FIG. 1. In theaspect shown in FIG. 1, a holder and temperature sensor 102 is installedand a temperature of the surface of the substrate 103 can be monitored.However, this configuration is not essential. In the aspect shown inFIG. 1, the longitudinal direction of the measurement sample 100 is adirection shown with an arrow in the drawing, and is a longitudinaldirection of a magnetic tape from which the measurement sample is cutout. In the invention and the specification, the description regarding“parallel” includes a range of errors allowed in the technical field ofthe invention. For example, the range of errors means a range of lessthan ±10° from an exact parallel state, and the error from the exactparallel state is preferably within ±5° and more preferably within ±3°.In addition, as a pendulum 107 (see FIG. 2), a pendulum formed of amaterial having properties of being adsorbed to a magnet (for example,formed of metal or formed of an alloy) is used.

The temperature of the surface of the substrate 103 on which themeasurement sample 100 is placed is set to 80° C. by increasing thetemperature at a rate of temperature increase equal to or lower than 5°C./min (arbitrary rate of temperature increase may be set, as long as itis equal to or lower than 5° C./min), and the pendulum movement isstarted (induce initial vibration) by releasing adsorption between thependulum 107 and a magnet 106. An example of a state of the pendulum 107which performs the pendulum movement (state seen from the side) is shownin FIG. 2. In the aspect shown in FIG. 2, in the pendulumviscoelasticity tester, the pendulum movement is started by stopping(switching off) the electricity to the magnet (electromagnet) 106disposed on the lower side of the sample stage to release theadsorption, and the pendulum movement is stopped by restarting(switching on) the electricity to the electromagnet to cause thependulum 107 to be adsorbed to the magnet 106. As shown in FIG. 2,during the pendulum movement, the pendulum 107 reciprocates theamplitude. From a result obtained by monitoring displacement of thependulum with a displacement sensor 108 while the pendulum reciprocatesthe amplitude, a displacement-time curve in which a vertical axisindicates the displacement and a horizontal axis indicates the elapsedtime is obtained. An example of the displacement-time curve is shown inFIG. 3. FIG. 3 schematically shows correspondence between the state ofthe pendulum 107 and the displacement-time curve. The stop (adsorption)and the pendulum movement are repeated at a regular measurementinterval, the logarithmic decrement A (no unit) is acquired from thefollowing Expression by using a displacement-time curve obtained in themeasurement interval after 10 minutes or longer (may be arbitrary time,as long as it is 10 minutes or longer) has elapsed, and this value isset as logarithmic decrement of the surface of the magnetic layer of themagnetic tape. The adsorption time of the first adsorption is set as 1second or longer (may be arbitrary time, as long as it is 1 second orlonger), and the interval between the adsorption stop and the adsorptionstart is set as 6 seconds or longer (may be arbitrary time, as long asit is 6 seconds or longer). The measurement interval is an interval ofthe time from the adsorption start and the next adsorption start. Inaddition, humidity of an environment in which the pendulum movement isperformed, may be arbitrary relative humidity, as long as the relativehumidity is 40% to 70%.

$\Delta = \frac{{\ln( \frac{A_{1}}{A_{2}} )} + {\ln( \frac{A_{2}}{A_{3}} )} + {\ldots\mspace{14mu}{\ln( \frac{A_{n}}{A_{n + 1}} )}}}{n}$

In the displacement-time curve, an interval between a point of theminimum displacement and a point of the next minimum displacement is setas a period of a wave. n indicates the number of waves included in thedisplacement-time curve in the measurement interval, and An indicatesthe minimum displacement and maximum displacement of the n-th wave. InFIG. 3, an interval between the minimum displacement of the n-th waveand the next minimum displacement is shown as Pn (for example, P₁regarding the first wave, P₂ regarding the second wave, and P₃ regardingthe third wave). In the calculation of the logarithmic decrement, adifference (in Expression A_(n+1), in the displacement-time curve shownin FIG. 3, A₄) between the minimum displacement and the maximumdisplacement appearing after the n-th wave is also used, but a partwhere the pendulum 107 stops (adsorption) after the maximum displacementis not used in the counting of the number of waves. In addition, a partwhere the pendulum 107 stops (adsorption) before the maximumdisplacement is not used in the counting of the number of waves, either.Accordingly, the number of waves is 3 (n=3) in the displacement-timecurve shown in FIG. 3.

The inventors have considered that the logarithmic decrement is an indexfor the amount of pressure sensitive adhesive components separated fromthe magnetic tape, in a case where the TMR head comes into contact withthe magnetic tape and slides on the magnetic tape, and interposedbetween the magnetic tape and the TMR head. The inventors haveconsidered that, as a larger amount of the pressure sensitive adhesivecomponents is present, adhesiveness between the magnetic tape and theTMR head increases, and this disturb smooth sliding between the magnetictape and the TMR head (sliding properties are deteriorated). Withrespect to this, the inventors have considered that, in the magnetictape included in the magnetic tape device, a state where the logarithmicdecrement is equal to or smaller than 0.050, that is, a state where theamount of the pressure sensitive adhesive components is decreasedcontributes to smooth sliding between the magnetic tape and the TMRhead. As a result, the inventors have surmised that it is possible toprevent occurrence of short circuit due to damage on the TMR head due tothe sliding on the magnetic tape having the magnetic layer surfaceroughness Ra of 2.0 nm and excellent smoothness of the surface of themagnetic layer.

The details of the pressure sensitive adhesive components are not clear.The inventors have surmised that the pressure sensitive adhesivecomponents may be derived from a resin used as a binding agent. Thespecific description is as follows. As a binding agent, various resinscan be used as will be described later in detail. The resin is a polymer(including a homopolymer or a copolymer) of two or more polymerizablecompounds and generally also includes a component having a molecularweight which is smaller than an average molecular weight (hereinafter,referred to as a “binding agent component having a low molecularweight”). The inventors have surmised that the binding agent componenthaving a low molecular weight which is separated from the magnetic tapeat the time of sliding between the magnetic tape and the TMR head andinterposed between the magnetic tape and the TMR head may cause adecrease in sliding properties. The inventors have surmised that, thebinding agent component having a low molecular weight may have pressuresensitive adhesive properties and the logarithmic decrement acquired bya pendulum viscoelasticity test may be an index for the amount ofbinding agent components having a low molecular weight separated fromthe magnetic tape at the time of the sliding between the magnetic tapeand the TMR head. In one aspect, the magnetic layer is formed byapplying a magnetic layer forming composition including a curing agentin addition to ferromagnetic powder and a binding agent onto anon-magnetic support directly or with another layer interposedtherebetween, and performing curing process. With the curing processhere, it is possible to allow a curing reaction (crosslinking reaction)between the binding agent and the curing agent. However, although thereason thereof is not clear, the inventors have considered that thebinding agent component having a low molecular weight may have poorreactivity regarding the curing reaction. Accordingly, the inventorshave surmised that the binding agent component having a low molecularweight which hardly remains in the magnetic layer and is easilyseparated from the magnetic layer may be one of reasons for that thebinding agent component having a low molecular weight is interposedbetween the magnetic tape and the TMR head at the time of the slidingbetween the magnetic tape and the TMR head.

However, the above-mentioned description is merely a surmise of theinventors and the invention is not limited thereto.

Hereinafter, the magnetic tape device will be described morespecifically. A “decrease in resistance value of the TMR head” describedbelow is a significant decrease in resistance value of the TMR headoccurring in a case of reading a servo pattern using the TMR head, inthe magnetic tape device in which the TMR head is mounted as a servohead, otherwise not noted. In the invention and the specification, thepowder means an aggregate of a plurality of particles. For example, theferromagnetic powder means an aggregate of a plurality of ferromagneticparticles. The aggregate of the plurality of particles not only includesan aspect in which particles configuring the aggregate directly comeinto contact with each other, and also includes an aspect in which abinding agent or an additive which will be described later is interposedbetween the particles. A term “particles” is also used for describingthe powder.

Magnetic Tape

Magnetic Layer Surface Roughness Ra

The center line average surface roughness Ra measured regarding thesurface of the magnetic layer of the magnetic tape (magnetic layersurface roughness Ra) is equal to or smaller than 2.0 nm. This point cancontribute to the reading of the servo pattern a high SNR in themagnetic tape device. From a viewpoint of further increasing the SNR,the magnetic layer surface roughness Ra is preferably equal to orsmaller than 1.9 nm, more preferably equal to or smaller than 1.8 nm,even more preferably equal to or smaller than 1.7 nm, still preferablyequal to or smaller than 1.6 nm, and still more preferably equal to orsmaller than 1.5 nm. In addition, the magnetic layer surface roughnessRa can be, for example, equal to or greater than 1.0 nm or equal to orgreater than 1.2 nm. However, from a viewpoint of increasing the SNR, alow magnetic layer surface roughness Ra is preferable, and thus, themagnetic layer surface roughness Ra may be lower than the lower limitexemplified above.

The center line average surface roughness Ra measured regarding thesurface of the magnetic layer of the magnetic tape in the invention andthe specification is a value measured with an atomic force microscope(AFM) in a region having an area of 40 μm×40 μm of the surface of themagnetic layer. As an example of the measurement conditions, thefollowing measurement conditions can be used. The magnetic layer surfaceroughness Ra shown in examples which will be described later is a valueobtained by the measurement under the following measurement conditions.In the invention and the specification, the “surface of the magneticlayer” of the magnetic tape is identical to the surface of the magnetictape on the magnetic layer side.

The measurement is performed regarding the region of 40 μm×40 μm of thearea of the surface of the magnetic layer of the magnetic tape with anAFM (Nanoscope 4 manufactured by Veeco Instruments, Inc.) in a tappingmode. RTESP-300 manufactured by BRUKER is used as a probe, a scan speed(probe movement speed) is set as 40 μm/sec, and a resolution is set as512 pixel×512 pixel.

The magnetic layer surface roughness Ra can be controlled by awell-known method. For example, the magnetic layer surface roughness Racan be changed in accordance with the size of various powders includedin the magnetic layer or manufacturing conditions of the magnetic tape.Thus, by adjusting one or more of these, it is possible to obtain amagnetic tape having the magnetic layer surface roughness Ra equal to orsmaller than 2.0 nm.

Logarithmic Decrement

The logarithmic decrement acquired by a pendulum viscoelasticity testperformed regarding the surface of the magnetic layer of the magnetictape is equal to or smaller than 0.050. Accordingly, it is possible toprevent a decrease in resistance value of the TMR head. The logarithmicdecrement is preferably equal to or smaller than 0.048, more preferablyequal to or smaller than 0.045, and even more preferably equal to orsmaller than 0.040, from a viewpoint of further preventing a decrease inresistance value of the TMR head. Meanwhile, from a viewpoint ofpreventing a decrease in resistance value of the TMR head, it ispreferable that the logarithmic decrement is low, and thus, a lowerlimit value is not particularly limited. The logarithmic decrement canbe, for example, equal to or greater than 0.010 or equal to or greaterthan 0.015. However, the logarithmic decrement may be smaller than theexemplified value. A specific aspect of a method for adjusting thelogarithmic decrement will be described later.

Magnetic Cluster Area Ratio Sdc/Sac

The magnetic cluster area ratio Sdc/Sac of the magnetic tape is 0.80 to1.30. It is thought that the magnetic cluster area ratio Sdc/Sac is avalue which may be an index showing a state of ferromagnetic powderpresent in the magnetic layer. Specifically, in the magnetic layer ofthe magnetic tape in the AC demagnetization state, each ferromagneticparticle faces a random direction and the total amount of magnetizationis close to zero. Accordingly, each ferromagnetic particle can besubstantially present in a state of a primary particle. Thus, a size ofthe magnetic cluster in the AC demagnetization state (specifically,average area Sac which will be described later in detail) can be a valuewhich does not vary depending on an aggregation state of ferromagneticparticles of the magnetic layer. Meanwhile, a size of the magneticcluster in the DC demagnetization (a degree of a magnetic field was setas zero, after applying DC magnetic field) state (specifically, averagearea Sdc which will be described later in detail) corresponds to a sizeof an aggregate of the ferromagnetic particles and varies depending on adegree of aggregation of the ferromagnetic particles in the magneticlayer. As the ferromagnetic particles are aggregated, the value thereoftends to increase. Therefore, it is thought that a small differencebetween the Sdc and Sac means that the aggregation of the particles ofthe ferromagnetic powder is prevented. For details of this point, adescription disclosed in paragraphs 0014 to 0017 of JP2007-294084A canbe referred to, for example. It is thought that the magnetic clusterarea ratio Sdc/Sac which is 0.80 to 1.30 contributes to an increase inSNR at the time of reading a servo pattern written in the magnetic layerby the TMR head. From a viewpoint of further increasing the SNR, themagnetic cluster area ratio Sdc/Sac is preferably equal to or smallerthan 1.28, more preferably equal to or smaller than 1.25, even morepreferably equal to or smaller than 1.20, still preferably equal to orsmaller than 1.15, still more preferably equal to or smaller than 1.10,still even more preferably equal to or smaller than 1.05, and stillfurther more preferably equal to or smaller than 1.00. A lower limit ofthe measurement value of the magnetic cluster area ratio Sdc/Sac is 0.80as known in the related art (for example, see a paragraph 0018 ofJP2007-294084A).

The magnetic cluster area ratio Sdc/Sac of the invention and thespecification is a value obtained by the following method by measurementperformed using a magnetic force microscope (MFM).

Two samples cut out from the same magnetic tape are prepared. One sampleis used for measuring the Sac by performing the AC demagnetization andthe other sample is used for measuring the Sdc by allowing the DCdemagnetization.

The Sac is a value obtained by the following method.

With a magnetic force microscope, a magnetic force image is obtained ina square area having a length of a side of 5 μm (5 μm×5 μm) of a surfaceof a magnetic layer of the sample regarding which demagnetization isperformed in the alternating magnetic field (alternating current (AC)demagnetization). An area of the magnetic force image is calculatedafter performing noise removing and hole filling treatment regarding theobtained magnetic force image, by using well-known image analysissoftware. The above operation is performed with respect to magneticforce images of arbitrarily selected 10 different points of the surfaceof the magnetic layer, and an arithmetical mean (average area) of theareas of the magnetic force image is calculated. The average areacalculated as described above is set as the Sac.

The Sdc is a value obtained by the following method.

The sample was subjected to direct current (DC) demagnetization atapplying magnetic field of 796 kA/m (10 kOe), and then, a magnetic forceimage having a square area having a length of a side of 5 μm (5 μm×5 μm)of the sample subjected to the DC demagnetization is obtained by using amagnetic force microscope. An area of the magnetic force image iscalculated after performing noise removing and hole filling treatmentregarding the obtained magnetic force image, by using well-known imageanalysis software. The above operation is performed with respect tomagnetic force images of arbitrarily selected 10 different points of thesurface of the magnetic layer, and an arithmetical mean (average area)of the areas of the magnetic force image is calculated. The average areacalculated as described above is set as the Sdc.

The ratio of the Sdc and Sac obtained as described above (Sdc/Sac) isset as the magnetic cluster area ratio Sdc/Sac.

The obtaining of the magnetic force image with a magnetic forcemicroscope is performed by using a commercially available magnetic forcemicroscope or a magnetic force microscope having a well-knownconfiguration in a frequency modulation (FM) mode. As a probe of themagnetic force microscope, SSS-MFMR (nominal radius of curvature of 15nm) manufactured by Nano World AG can be used, for example. A distancebetween the surface of the magnetic layer and a distal end of the probeat the time of the magnetic force microscope observation is 20 to 50 nm.In addition, well-known analysis software or analysis software using awell-known arithmetic expression can be used as the image analysissoftware.

The magnetic cluster area ratio Sdc/Sac can be controlled to be 0.80 to1.30 by preventing the aggregation of the ferromagnetic particles of themagnetic layer. As a method for preventing the aggregation, thefollowing method can be used, for example.

As a binding agent included in the magnetic layer, a binding agenthaving high affinity with a solvent used in preparation of a magneticlayer forming composition is used.

Dispersion conditions at the time of preparing the magnetic layerforming composition are adjusted.

A process for crushing aggregation of ferromagnetic particles isperformed after arbitrarily applying the magnetic layer formingcomposition onto the non-magnetic support through a non-magnetic layer.

For the above point and other controlling methods, descriptionsdisclosed in paragraphs 0012 and 0032 and examples of JP4001532B, andparagraphs 0024 to 0026, 0028, 0029, 0105 and 0106, and examples ofJP2007-294084A can be referred to, for example.

In addition, regarding ferromagnetic powder having low saturationmagnetization σs, ferromagnetic particles configuring the ferromagneticpowder are hardly aggregated, and even in a case where the ferromagneticparticles are aggregated, the aggregation tends to be easily crushed.Accordingly, a method of forming a magnetic layer by using ferromagneticpowder having low saturation magnetization us can be a method ofcontrolling the magnetic cluster area ratio Sdc/Sac.

For example, the magnetic cluster area ratio Sdc/Sac can be controlledto be 0.80 to 1.30 by arbitrarily combining one or two or more variousmethods described above.

In one aspect, the Sdc and Sac are respectively preferably 3,000 to50,000 nm², more preferably 3,000 to 35,000 nm², and even morepreferably 3,000 to 20,000 nm². The Sdc and Sac are respectivelypreferably equal to or greater than 3,000 nm², from a viewpoint ofstability of magnetization, and are respectively preferably equal to orsmaller than 50,000 nm², from a viewpoint of increasing resolution atthe time of high-density recording. The Sac can be controlled by usingthe average particle size of the ferromagnetic powder used for formingthe magnetic layer, and the Sdc can be controlled by preventingaggregation of the ferromagnetic particles of the magnetic layer. Themethod of preventing the aggregation is as described above.

Next, the magnetic layer and the like included in the magnetic tape willbe described more specifically.

Magnetic Layer

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer,ferromagnetic powder normally used in the magnetic layer of variousmagnetic recording media can be used. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density of the magnetic tape. Fromthis viewpoint, ferromagnetic powder having an average particle sizeequal to or smaller than 50 nm is preferably used as the ferromagneticpowder. Meanwhile, the average particle size of the ferromagnetic powderis preferably equal to or greater than 10 nm, from a viewpoint ofstability of magnetization.

As a preferred specific example of the ferromagnetic powder,ferromagnetic hexagonal ferrite powder can be used. An average particlesize of the ferromagnetic hexagonal ferrite powder is preferably 10 nmto 50 nm and more preferably 20 nm to 50 nm, from a viewpoint ofimprovement of recording density and stability of magnetization. Fordetails of the ferromagnetic hexagonal ferrite powder, descriptionsdisclosed in paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134to 0136 of JP2011-216149A, and paragraphs 0013 to 0030 of JP2012-204726Acan be referred to, for example.

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. An average particle size ofthe ferromagnetic metal powder is preferably 10 nm to 50 nm and morepreferably 20 nm to 50 nm, from a viewpoint of improvement of recordingdensity and stability of magnetization. For details of the ferromagneticmetal powder, descriptions disclosed in paragraphs 0137 to 0141 ofJP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A can bereferred to, for example.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on printing paperso that the total magnification of 500,000 to obtain an image ofparticles configuring the powder. A target particle is selected from theobtained image of particles, an outline of the particle is traced with adigitizer, and a size of the particle (primary particle) is measured.The primary particle is an independent particle which is not aggregated.

The measurement described above is performed regarding 500 particlesarbitrarily extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted.

As a method of collecting a sample powder from the magnetic tape inorder to measure the particle size, a method disclosed in a paragraph of0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter, and an average plate ratio is an arithmeticalmean of (maximum long diameter/thickness or height). In a case of thedefinition (3), the average particle size is an average diameter (alsoreferred to as an average particle diameter).

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50 to 90 mass % and more preferably 60 to90 mass %. The components other than the ferromagnetic powder of themagnetic layer are at least a binding agent and one or more kinds ofadditives may be arbitrarily included. A high filling percentage of theferromagnetic powder in the magnetic layer is preferable from aviewpoint of improvement recording density.

Binding Agent

The magnetic tape is a coating type magnetic tape, and the magneticlayer includes a binding agent together with the ferromagnetic powder.As the binding agent, one or more kinds of resin is used. The resin maybe a homopolymer or a copolymer. As the binding agent, various resinsnormally used as a binding agent of the coating type magnetic recordingmedium can be used. For example, as the binding agent, a resin selectedfrom a polyurethane resin, a polyester resin, a polyamide resin, a vinylchloride resin, an acrylic resin obtained by copolymerizing styrene,acrylonitrile, or methyl methacrylate, a cellulose resin such asnitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylalresin such as polyvinyl acetal or polyvinyl butyral can be used alone ora plurality of resins can be mixed with each other to be used. Amongthese, a polyurethane resin, an acrylic resin, a cellulose resin, and avinyl chloride resin are preferable. These resins can be used as thebinding agent even in the non-magnetic layer and/or a back coating layerwhich will be described later. For the binding agent described above,description disclosed in paragraphs 0028 to 0031 of JP2010-24113A can bereferred to. In addition, as described above, it is preferable to use abinding agent having high affinity with a solvent, from a viewpoint ofpreventing aggregation of the ferromagnetic particles of the magneticlayer.

An average molecular weight of the resin used as the binding agent canbe, for example, 10,000 to 200,000 as a weight-average molecular weight.The weight-average molecular weight of the invention and thespecification is a value obtained by performing polystyrene conversionof a value measured by gel permeation chromatography (GPC). As themeasurement conditions, the following conditions can be used. Theweight-average molecular weight shown in examples which will bedescribed later is a value obtained by performing polystyrene conversionof a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In addition, a curing agent can also be used together with the bindingagent. As the curing agent, in one aspect, a thermosetting compoundwhich is a compound in which a curing reaction (crosslinking reaction)proceeds due to heating can be used, and in another aspect, aphotocurable compound in which a curing reaction (crosslinking reaction)proceeds due to light irradiation can be used. At least a part of thecuring agent is included in the magnetic layer in a state of beingreacted (crosslinked) with other components such as the binding agent,by proceeding the curing reaction in the magnetic layer forming step.The preferred curing agent is a thermosetting compound, polyisocyanateis suitable. For details of the polyisocyanate, descriptions disclosedin paragraphs 0124 and 0125 of JP2011-216149A can be referred to, forexample. The amount of the curing agent can be, for example, 0 to 80.0parts by mass with respect to 100.0 parts by mass of the binding agentin the magnetic layer forming composition, and is preferably 50.0 to80.0 parts by mass, from a viewpoint of improvement of strength of eachlayer such as the magnetic layer.

Other Components

The magnetic layer may include one or more kinds of additives, ifnecessary, together with the various components described above. As theadditives, a commercially available product can be suitably selected andused according to the desired properties. Alternatively, a compoundsynthesized by a well-known method can be used as the additives. As theadditives, the curing agent described above is used as an example. Inaddition, examples of the additive which can be included in the magneticlayer include a non-magnetic filler, a lubricant, a dispersing agent, adispersing assistant, an antibacterial agent, an antistatic agent, anantioxidant, and carbon black. The non-magnetic filler is identical tothe non-magnetic powder. As the non-magnetic filler, a non-magneticfiller (hereinafter, referred to as a “projection formation agent”)which can function as a projection formation agent which formsprojections suitably protruded from the surface of the magnetic layer,and a non-magnetic filler (hereinafter, referred to as an “abrasive”)which can function as an abrasive can be used.

Non-Magnetic Filler

As the projection formation agent which is one aspect of thenon-magnetic filler, various non-magnetic powders normally used as aprojection formation agent can be used. These may be inorganicsubstances or organic substances. In one aspect, from a viewpoint ofhomogenization of friction properties, particle size distribution of theprojection formation agent is not polydispersion having a plurality ofpeaks in the distribution and is preferably monodisperse showing asingle peak. From a viewpoint of availability of monodisperse particles,the projection formation agent is preferably powder of inorganicsubstances (inorganic powder). Examples of the inorganic powder includepowder of inorganic oxide such as metal oxide, metal carbonate, metalsulfate, metal nitride, metal carbide, and metal sulfide, and powder ofinorganic oxide is preferable. The projection formation agent is morepreferably colloidal particles and even more preferably inorganic oxidecolloidal particles. In addition, from a viewpoint of availability ofmonodisperse particles, the inorganic oxide configuring the inorganicoxide colloidal particles are preferably silicon dioxide (silica). Theinorganic oxide colloidal particles are more preferably colloidal silica(silica colloidal particles). In the invention and the specification,the “colloidal particles” are particles which are not precipitated anddispersed to generate a colloidal dispersion, in a case where 1 g of theparticles is added to 100 mL of at least one organic solvent of at leastmethyl ethyl ketone, cyclohexanone, toluene, or ethyl acetate, or amixed solvent including two or more kinds of the solvent described aboveat an arbitrary mixing ratio. The average particle size of the colloidalparticles is a value obtained by a method disclosed in a paragraph 0015of JP2011-048878A as a measurement method of an average particlediameter. In addition, in another aspect, the projection formation agentis preferably carbon black.

An average particle size of the projection formation agent is, forexample, 30 to 300 nm and is preferably 40 to 200 nm.

The abrasive which is another aspect of the non-magnetic filler ispreferably non-magnetic powder having Mohs hardness exceeding 8 and morepreferably non-magnetic powder having Mohs hardness equal to or greaterthan 9. A maximum value of Mohs hardness is 10 of diamond. Specifically,powders of alumina (Al₂O₃), silicon carbide, boron carbide (B₄C), SiO₂,TiC, chromium oxide (Cr₂O₃), cerium oxide, zirconium oxide (ZrO₂), ironoxide, diamond, and the like can be used, and among these, aluminapowder such as α-alumina and silicon carbide powder are preferable. Inaddition, regarding the particle size of the abrasive, a specificsurface area which is an index for the particle size is, for example,equal to or greater than 14 m²/g, and is preferably 16 m²/g and morepreferably 18 m²/g. Further, the specific surface area of the abrasivecan be, for example, equal to or smaller than 40 m²/g. The specificsurface area is a value obtained by a nitrogen adsorption method (alsoreferred to as a Brunauer-Emmett-Teller (BET) 1 point method), and is avalue measured regarding primary particles. Hereinafter, the specificsurface area obtained by such a method is also referred to as a BETspecific surface area.

In addition, from a viewpoint that the projection formation agent andthe abrasive can exhibit the functions thereof in more excellent manner,the content of the projection formation agent of the magnetic layer ispreferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 partsby mass with respect to 100.0 parts by mass of the ferromagnetic powder.Meanwhile, the content of the magnetic layer is preferably 1.0 to 20.0parts by mass, more preferably 3.0 to 15.0 parts by mass, and even morepreferably 4.0 to 10.0 parts by mass with respect to 100.0 parts by massof the ferromagnetic powder.

As an example of the additive which can be used in the magnetic layerincluding the abrasive, a dispersing agent disclosed in paragraphs 0012to 0022 of JP2013-131285A can be used as a dispersing agent forimproving dispersibility of the abrasive of the magnetic layer formingcomposition. It is preferable to improve dispersibility of thenon-magnetic filler such as an abrasive in the magnetic layer formingcomposition, in order to decrease the magnetic layer surface roughnessRa.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic tape mayinclude a magnetic layer directly on a non-magnetic support, or mayinclude a non-magnetic layer including non-magnetic powder and a bindingagent between the non-magnetic support and the magnetic layer. Thenon-magnetic powder used in the non-magnetic layer may be powder ofinorganic substances or powder of organic substances. In addition,carbon black and the like can be used. Examples of the inorganicsubstances include metal, metal oxide, metal carbonate, metal sulfate,metal nitride, metal carbide, and metal sulfide. These non-magneticpowder can be purchased as a commercially available product or can bemanufactured by a well-known method. For details thereof, descriptionsdisclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referredto. For carbon black which can be used in the non-magnetic layer,descriptions disclosed in paragraphs 0040 and 0041 of JP2010-24113A canbe referred to. The content (filling percentage) of the non-magneticpowder of the non-magnetic layer is preferably 50 to 90 mass % and morepreferably 60 to 90 mass %.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

The non-magnetic layer of the magnetic tape also includes asubstantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support will be described. As the non-magneticsupport (hereinafter, also simply referred to as a “support”),well-known components such as polyethylene terephthalate, polyethylenenaphthalate, polyamide, polyamide imide, aromatic polyamide subjected tobiaxial stretching are used. Among these, polyethylene terephthalate,polyethylene naphthalate, and polyamide are preferable. Coronadischarge, plasma treatment, easy-bonding treatment, or heatingtreatment may be performed with respect to these supports in advance.

Back Coating Layer

The magnetic tape can also include a back coating layer includingnon-magnetic powder and a binding agent on a surface side of thenon-magnetic support opposite to the surface provided with the magneticlayer. The back coating layer preferably includes any one or both ofcarbon black and inorganic powder. In regards to the binding agentincluded in the back coating layer and various additives which can bearbitrarily included in the back coating layer, a well-known technologyregarding the treatment of the magnetic layer and/or the non-magneticlayer can be applied.

Various Thickness

A thickness of the non-magnetic support is preferably 3.00 to 6.00 μm.

A thickness of the magnetic layer is preferably equal to or smaller than0.15 μm and more preferably equal to or smaller than 0.10 μm, from aviewpoint of realization of high-density recording required in recentyears. The thickness of the magnetic layer is even more preferably 0.01to 0.10 μM. The magnetic layer may be at least single layer, themagnetic layer may be separated into two or more layers having differentmagnetic properties, and a configuration of a well-known multilayeredmagnetic layer can be applied. A thickness of the magnetic layer in acase where the magnetic layer is separated into two or more layers is atotal thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μmand is preferably 0.10 to 1.00 μm.

Meanwhile, the magnetic tape is normally used to be accommodated andcirculated in a magnetic tape cartridge. In order to increase recordingcapacity for 1 reel of the magnetic tape cartridge, it is desired toincrease a total length of the magnetic tape accommodated in 1 reel ofthe magnetic tape cartridge. In order to increase the recordingcapacity, it is necessary that the magnetic tape is thinned(hereinafter, referred to as “thinning”). As one method of thinning themagnetic tape, a method of decreasing a total thickness of a magneticlayer and a non-magnetic layer of a magnetic tape including thenon-magnetic layer and the magnetic layer on a non-magnetic support inthis order is used. In a case where the magnetic tape includes anon-magnetic layer, the total thickness of the magnetic layer and thenon-magnetic layer is preferably equal to or smaller than 1.80 μm, morepreferably equal to or smaller than 1.50 μm, and even more preferablyequal to or smaller than 1.10 μm, from a viewpoint of thinning themagnetic tape. In addition, the total thickness of the magnetic layerand the non-magnetic layer can be, for example, equal to or greater than0.10 μm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.90 μm and even more preferably 0.10 to 0.70 μm.

The thicknesses of various layers of the magnetic tape and thenon-magnetic support can be acquired by a well-known film thicknessmeasurement method. As an example, a cross section of the magnetic tapein a thickness direction is, for example, exposed by a well-known methodof ion beams or microtome, and the exposed cross section is observedwith a scanning electron microscope. In the cross section observation,various thicknesses can be acquired as a thickness acquired at oneposition of the cross section in the thickness direction, or anarithmetical mean of thicknesses acquired at a plurality of positions oftwo or more positions, for example, two positions which are arbitrarilyextracted. In addition, the thickness of each layer may be acquired as adesigned thickness calculated according to the manufacturing conditions.

Manufacturing Method

Preparation of Each Layer Forming Composition

Each composition for forming the magnetic layer, the non-magnetic layer,or the back coating layer normally includes a solvent, together withvarious components described above. As the solvent, various organicsolvents generally used for manufacturing a coating type magneticrecording medium can be used. Among those, from a viewpoint ofsolubility of the binding agent normally used in the coating typemagnetic recording medium, each layer forming composition preferablyincludes one or more ketone solvents such as acetone, methyl ethylketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone,isophorone, and tetrahydrofuran. The amount of the solvent of each layerforming composition is not particularly limited, and can be set to bethe same as that of each layer forming composition of a typical coatingtype magnetic recording medium. In addition, steps of preparing eachlayer forming composition generally include at least a kneading step, adispersing step, and a mixing step provided before and after thesesteps, if necessary. Each step may be divided into two or more stages.All of raw materials used in the invention may be added at an initialstage or in a middle stage of each step. In addition, each raw materialmay be separately added in two or more steps. For example, a bindingagent may be separately added in a kneading step, a dispersing step, anda mixing step for adjusting viscosity after the dispersion. In amanufacturing step of the magnetic tape, a well-known manufacturingtechnology of the related art can be used in a part of the step or inthe entire step. In the kneading step, an open kneader, a continuouskneader, a pressure kneader, or a kneader having a strong kneading forcesuch as an extruder is preferably used. The details of the kneadingprocesses of these kneaders are disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-79274A (JP-H01-79274A). In addition, inorder to disperse each layer forming composition, glass beads and/orother beads can be used. As such dispersion beads, zirconia beads,titania beads, and steel beads which are dispersion beads having highspecific gravity are preferable. These dispersion beads are preferablyused by optimizing a bead diameter and a filling percentage. As adispersing machine, a well-known dispersing machine can be used. Eachlayer forming composition may be filtered by a well-known method beforeperforming the coating step. The filtering can be performed by using afilter, for example. As the filter used in the filtering, a filterhaving a hole diameter of 0.01 to 3 μm can be used, for example. Inaddition, as described above, it is also preferable that the dispersionconditions for controlling the magnetic cluster area ratio Sdc/Sac areadjusted. An increase in dispersion time, a decrease in diameter ofdispersion beads used in the dispersion, an increase in fillingpercentage of the dispersion beads, and the like are preferable, from aviewpoint of preventing aggregation of the ferromagnetic particles ofthe magnetic layer. For details of the manufacturing method of themagnetic tape, a description disclosed in paragraphs 0051 to 0057 ofJP2010-24113A can also be referred to.

Coating Step, Cooling Step, Heating and Drying Step, BurnishingTreatment Step, and Curing Step

The magnetic layer can be formed by directly applying the magnetic layerforming composition onto the non-magnetic support or performingmultilayer coating of the magnetic layer forming composition with thenon-magnetic layer forming composition in order or at the same time. Fordetails of the coating for forming each layer, a description disclosedin a paragraph 0066 of JP2010-231843A can be referred to.

In a preferred aspect, a magnetic layer can be formed through a magneticlayer forming step including a coating step of applying a magnetic layerforming composition including ferromagnetic powder, a binding agent, acuring agent, and a solvent onto a non-magnetic support directly or withanother layer interposed therebetween, to form a coating layer, aheating and drying step of drying the coating layer by a heatingprocess, and a curing step of performing a curing process with respectto the coating layer. The magnetic layer forming step preferablyincludes a cooling step of cooling the coating layer between the coatingstep and the heating and drying step, and more preferably includes aburnishing treatment step of performing a burnishing treatment withrespect to the surface of the coating layer between the heating anddrying step and the curing step.

The inventors have thought that it is preferable that the cooling stepand the burnishing treatment step in the magnetic layer forming step, inorder to set the logarithmic decrement to be equal to or smaller than0.050. More specific description is as follows.

The inventors have surmised that performing the cooling step of coolingthe coating layer between the coating step and the heating and dryingstep contributes to causing pressure sensitive adhesive componentseparated from the magnetic tape (specifically, surface of the magneticlayer) in a case where the TMR head comes into contact with and slideson the magnetic tape, to be localized in the surface and/or a surfacelayer part in the vicinity of the surface of the coating layer. Theinventors have surmised that this is because the pressure sensitiveadhesive component at the time of solvent volatilization in the heatingand drying step is easily moved to the surface and/or the surface layerpart of the coating layer, by cooling the coating layer of the magneticlayer forming composition before the heating and drying step. However,the reason thereof is not clear. In addition, the inventors have thoughtthat the pressure sensitive adhesive component can be removed byperforming the burnishing treatment with respect to the surface of thecoating layer in which the pressure sensitive adhesive component islocalized on the surface and/or surface layer part. The inventors havesurmised that performing the curing step after removing the pressuresensitive adhesive component contributes setting the logarithmicdecrement to be equal to or smaller than 0.050. However, this is merelya surmise, and the invention is not limited thereto.

As described above, multilayer coating of the magnetic layer formingcomposition can be performed with the non-magnetic layer formingcomposition in order or at the same time. In a preferred aspect, themagnetic tape can be manufactured by successive multilayer coating. Amanufacturing step including the successive multilayer coating can bepreferably performed as follows. The non-magnetic layer is formedthrough a coating step of applying a non-magnetic layer formingcomposition onto a non-magnetic support to form a coating layer, and aheating and drying step of drying the formed coating layer by a heatingprocess. In addition, the magnetic layer is formed through a coatingstep of applying a magnetic layer forming composition onto the formednon-magnetic layer to form a coating layer, and a heating and dryingstep of drying the formed coating layer by a heating process.

Hereinafter, a specific aspect of the manufacturing method of themagnetic tape will be described with reference to FIG. 4. However, theinvention is not limited to the following specific aspect.

FIG. 4 is a step schematic view showing a specific aspect of a step ofmanufacturing the magnetic tape including a non-magnetic layer and amagnetic layer in this order on one surface of a non-magnetic supportand including a back coating layer on the other surface thereof. In theaspect shown in FIG. 4, an operation of sending a non-magnetic support(elongated film) from a sending part and winding the non-magneticsupport around a winding part is continuously performed, and variousprocesses of coating, drying, and orientation are performed in each partor each zone shown in FIG. 4, and thus, it is possible to sequentiallyform a non-magnetic layer and a magnetic layer on one surface of therunning non-magnetic support by multilayer coating and to form a backcoating layer on the other surface thereof. Such a manufacturing methodcan be set to be identical to the manufacturing method normallyperformed for manufacturing a coating type magnetic recording medium,except for including a cooling zone in the magnetic layer forming stepand including the burnishing treatment step before the curing process.

The non-magnetic layer forming composition is applied onto thenon-magnetic support sent from the sending part in a first coating part(coating step of non-magnetic layer forming composition).

After the coating step, in a first heating process zone, the coatinglayer of the non-magnetic layer forming composition formed in thecoating step is heated after to dry the coating layer (heating anddrying step). The heating and drying step can be performed by causingthe non-magnetic support including the coating layer of the non-magneticlayer forming composition to pass through the heated atmosphere. Anatmosphere temperature of the heated atmosphere here can be, forexample, approximately 60° to 140°. Here, the atmosphere temperature maybe a temperature at which the solvent is volatilized and the coatinglayer is dried, and the atmosphere temperature is not limited to therange described above. In addition, the heated air may blow to thesurface of the coating layer. The points described above are alsoapplied to a heating and drying step of a second heating process zoneand a heating and drying step of a third heating process zone which willbe described later, in the same manner.

Next, in a second coating part, the magnetic layer forming compositionis applied onto the non-magnetic layer formed by performing the heatingand drying step in the first heating process zone (coating step ofmagnetic layer forming composition).

After the coating step, a coating layer of the magnetic layer formingcomposition formed in the coating step is cooled in a cooling zone(cooling step). For example, it is possible to perform the cooling stepby allowing the non-magnetic support on which the coating layer isformed on the non-magnetic layer to pass through a cooling atmosphere.An atmosphere temperature of the cooling atmosphere is preferably −10°C. to 0° C. and more preferably −5° C. to 0° C. The time for performingthe cooling step (for example, time while an arbitrary part of thecoating layer is delivered to and sent from the cooling zone(hereinafter, also referred to as a “staying time”)) is not particularlylimited. In a case where the staying time is long, the value oflogarithmic decrement tends to be increased. Thus, the staying time ispreferably adjusted by performing preliminary experiment if necessary,so that the logarithmic decrement equal to or smaller than 0.050 isrealized. In the cooling step, cooled air may blow to the surface of thecoating layer.

After that, in the aspect of performing the orientation process, whilethe coating layer of the magnetic layer forming composition is wet, anorientation process is performed with respect to the coating layer in anorientation zone. For the orientation process, a description disclosedin a paragraph 0052 of JP2010-24113A can be referred to. In addition, inone aspect, a process for preventing the aggregation of theferromagnetic particles included in the coating layer can be performedbefore and/or after the orientation process. As an example of such aprocess, a smoothing process can be used. The smoothing process is aprocess of applying shear to the coating layer by a smoother.

The coating layer after the orientation process is subjected to theheating and drying step in the second heating process zone.

Next, in the third coating part, a back coating layer formingcomposition is applied to a surface of the non-magnetic support on aside opposite to the surface where the non-magnetic layer and themagnetic layer are formed, to form a coating layer (coating step of backcoating layer forming composition). After that, the coating layer isheated and dried in the third heating process zone.

By doing so, it is possible to obtain the magnetic tape including thecoating layer of the magnetic layer forming composition heated and driedon the non-magnetic layer, on one surface side of the non-magneticsupport, and the back coating layer on the other surface side thereof.The magnetic tape obtained here becomes a magnetic tape product afterperforming various processes which will be described later.

The obtained magnetic tape is wound around the winding part, and cut(slit) to have a size of a magnetic tape product. The slitting isperformed by using a well-known cutter.

In the slit magnetic tape, the burnishing treatment is performed withrespect to the surface of the heated and dried coating layer of themagnetic layer forming composition, before performing the curing process(heating and light irradiation) in accordance with the types of thecuring agent included in the magnetic layer forming composition(burnishing treatment step between heating and drying step and curingstep). The inventors have surmised that removing the pressure sensitiveadhesive component transitioned to the surface and/or the surface layerpart of the coating layer cooled in the cooling zone by the burnishingtreatment contributes setting the logarithmic decrement to be equal toor smaller than 0.050. However, as described above, this is merely asurmise, and the invention is not limited thereto.

The burnishing treatment is treatment of rubbing a surface of atreatment target with a member (for example, a polishing tape, or agrinding tool such as a grinding blade or a grinding wheel), and can beperformed in the same manner as the well-known burnishing treatment formanufacturing a coating type magnetic recording medium. However, in therelated art, the burnishing treatment was not performed in a stagebefore the curing step, after performing the cooling step and theheating and drying step. With respect to this, the logarithmic decrementcan be equal to or smaller than 0.050 by performing the burnishingtreatment in the stage described above. This point was newly found bythe inventors.

The burnishing treatment can be preferably performed by performing oneor both of rubbing of the surface of the coating layer of the treatmenttarget by a polishing tape (polishing) and rubbing of the surface of thecoating layer of the treatment target by a grinding tool (grinding). Ina case where the magnetic layer forming composition includes anabrasive, it is preferable to use a polishing tape including at leastone of an abrasive having higher Mohs hardness than that of the abrasivedescribed above. As the polishing tape, a commercially available productmay be used and a polishing tape manufactured by a well-known method maybe used. As the grinding tool, a well-known blade such as a fixed blade,a diamond wheel, or a rotary blade, or a grinding blade can be used. Inaddition, a wiping treatment of wiping the surface of the coating layerrubbed by the polishing tape and/or the grinding tool with a wipingmaterial. For details of preferred polishing tape, grinding tool,burnishing treatment, and wiping treatment, descriptions disclosed inparagraphs 0034 to 0048, FIG. 1 and examples of JP1994-52544A(JP-H06-52544A) can be referred to. As the burnishing treatment isreinforced, the value of the logarithmic decrement tends to bedecreased. The burnishing treatment can be reinforced as an abrasivehaving high hardness is used as the abrasive included in the polishingtape, and can be reinforced, as the amount of the abrasive in thepolishing tape is increased. In addition, the burnishing treatment canbe reinforced as a grinding tool having high hardness is used as thegrinding tool. In regards to the burnishing treatment conditions, theburnishing treatment can be reinforced as a sliding speed between thesurface of the coating layer of the treatment target and a member (forexample, a polishing tape or a grinding tool) is increased. The slidingspeed can be increased by increasing one or both of a speed at which themember is moved, and a speed at which the magnetic tape of the treatmenttarget is moved.

After the burnishing treatment (burnishing treatment step), the curingprocess is performed with respect to the coating layer of the magneticlayer forming composition. In the aspect shown in FIG. 4, the coatinglayer of the magnetic layer forming composition is subjected to thesurface smoothing treatment, after the burnishing treatment and beforethe curing process. The surface smoothing treatment is preferablyperformed by a calender process. For details of the calender process,for example, description disclosed in a paragraph 0026 of JP2010-231843Acan be referred to. As the calender process is reinforced, the surfaceof the magnetic tape can be smoothened. The calender process isreinforced, as the surface temperature (calender temperature) of acalender roll is increased and/or as calender pressure is increased.

After that, the curing process according to the type of the curing agentincluded in the coating layer is performed with respect to the coatinglayer of the magnetic layer forming composition (curing step). Thecuring process can be performed by the process according to the type ofthe curing agent included in the coating layer, such as a heatingprocess or light irradiation. The curing process conditions are notparticularly limited, and the curing process conditions may be suitablyset in accordance with the list of the magnetic layer formingcomposition used in the coating layer formation, the type of the curingagent, and the thickness of the coating layer. For example, in a casewhere the coating layer is formed by using the magnetic layer formingcomposition including polyisocyanate as the curing agent, the curingprocess is preferably the heating process. In a case where the curingagent is included in a layer other than the magnetic layer, a curingreaction of the layer can also be promoted by the curing process here.Alternatively, the curing step may be separately provided. After thecuring step, the burnishing treatment may be further performed.

As described above, it is possible to obtain a magnetic tape included inthe magnetic tape device according to one aspect of the invention.However, the manufacturing method described above is merely an example,the logarithmic decrement, the magnetic cluster area ratio Sdc/Sac, andthe magnetic layer surface roughness Ra can be controlled to be inrespective ranges described above by an arbitrary method capable ofadjusting the logarithmic decrement, the magnetic cluster area ratioSdc/Sac, and the magnetic layer surface roughness Ra and such an aspectis also included in the invention.

Formation of Servo Pattern

A servo pattern is formed in the magnetic layer by magnetizing aspecific position of the magnetic layer with a servo pattern recordinghead (also referred to as a “servo write head”). A well-known technologyregarding a servo pattern of the magnetic layer of the magnetic tapewhich is well known can be applied for the shapes of the servo patternwith which the head tracking servo can be performed and the dispositionthereof in the magnetic layer. For example, as a head tracking servosystem, a timing-based servo system and an amplitude-based servo systemare known. The servo pattern of the magnetic layer of the magnetic tapemay be a servo pattern capable of allowing head tracking servo of anysystem. In addition, a servo pattern capable of allowing head trackingservo in the timing-based servo system and a servo pattern capable ofallowing head tracking servo in the amplitude-based servo system may beformed in the magnetic layer.

The magnetic tape described above is generally accommodated in amagnetic tape cartridge and the magnetic tape cartridge is mounted inthe magnetic tape device. In the magnetic tape cartridge, the magnetictape is generally accommodated in a cartridge main body in a state ofbeing wound around a reel. The reel is rotatably provided in thecartridge main body. As the magnetic tape cartridge, a single reel typemagnetic tape cartridge including one reel in a cartridge main body anda twin reel type magnetic tape cartridge including two reels in acartridge main body are widely used. In a case where the single reeltype magnetic tape cartridge is mounted in the magnetic tape device(drive) in order to record and/or reproduce data (magnetic signals) tothe magnetic tape, the magnetic tape is drawn from the magnetic tapecartridge and wound around the reel on the drive side. A servo head isdisposed on a magnetic tape transportation path from the magnetic tapecartridge to a winding reel. Sending and winding of the magnetic tapeare performed between a reel (supply reel) on the magnetic tapecartridge side and a reel (winding reel) on the drive side. In themeantime, the servo head comes into contact with and slides on thesurface of the magnetic layer of the magnetic tape, and accordingly, thereading of a servo pattern is performed by the servo head. With respectto this, in the twin reel type magnetic tape cartridge, both reels ofthe supply reel and the winding reel are provided in the magnetic tapecartridge. The magnetic tape according to one aspect of the inventionmay be accommodated in any of single reel type magnetic tape cartridgeand twin reel type magnetic tape cartridge. The configuration of themagnetic tape cartridge is well known.

Servo Head

The magnetic tape device includes the TMR head as the servo head. TheTMR head is a magnetic head including a tunnel magnetoresistance effecttype element (TMR element). The TMR element can play a role of detectinga change in leakage magnetic field from the magnetic tape as a change inresistance value (electric resistance) by using a tunnelmagnetoresistance effect, as a servo pattern reading element for readinga servo pattern formed in the magnetic layer of the magnetic tape. Byconverting the detected change in resistance value into a change involtage, the servo pattern can be read (servo signal can be reproduced).

As the TMR head included in the magnetic tape device, a TMR head havinga well-known configuration including a tunnel magnetoresistance effecttype element (TMR element) can be used. For example, for details of thestructure of the TMR head, materials of each unit configuring the TMRhead, and the like, well-known technologies regarding the TMR head canbe used.

The TMR head is a so-called thin film head. The TMR element included inthe TMR head at least includes two electrode layers, a tunnel barrierlayer, a free layer, and a fixed layer. The TMR head includes a TMRelement in a state where cross sections of these layers face a side of asurface sliding on the magnetic tape. The tunnel barrier layer ispositioned between the two electrode layers and the tunnel barrier layeris an insulating layer. Meanwhile, the free layer and the fixed layerare magnetic layers. The free layer is also referred to as amagnetization free layer and is a layer in which a magnetizationdirection changes depending on the external magnetic field. On the otherhand, the fixed layer is a layer in which a magnetization direction doesnot change depending on the external magnetic field. The tunnel barrierlayer (insulating layer) is positioned between the two electrodes,normally, and thus, even in a case where a voltage is applied, ingeneral, a current does not flow or does not substantially flow.However, a current (tunnel current) flows by the tunnel effect dependingon a magnetization direction of the free layer affected by a leakagemagnetic field from the magnetic tape. The amount of a tunnel currentflow changes depending on a relative angle of a magnetization directionof the fixed layer and a magnetization direction of the free layer, andas the relative angle decreases, the amount of the tunnel current flowincreases. A change in amount of the tunnel current flow is detected asa change in resistance value by the tunnel magnetoresistance effect. Byconverting the change in resistance value into a change in voltage, theservo pattern can be read. For an example of the configuration of theTMR head, a description disclosed in FIG. 1 of JP2004-185676A can bereferred to, for example. However, there is no limitation to the aspectshown in the drawing. FIG. 1 of JP2004-185676A shows two electrodelayers and two shield layers. Here, a TMR head having a configuration inwhich the shield layer serves as an electrode layer is also well knownand the TMR head having such a configuration can also be used. In theTMR head, a current (tunnel current) flows between the two electrodesand thereby changing electric resistance, by the tunnelmagnetoresistance effect. The TMR head is a magnetic head having a CPPstructure, and thus, a direction in which a current flows is atransportation direction of the magnetic tape. In the invention and thespecification, the description regarding “orthogonal” includes a rangeof errors allowed in the technical field of the invention. For example,the range of errors means a range of less than ±10° from an exactorthogonal state, and the error from the exact orthogonal state ispreferably within ±5° and more preferably within ±3°. A decrease inresistance value of the TMR head means a decrease in electric resistancemeasured by bringing an electric resistance measuring device intocontact with a wiring connecting two electrodes, and a decrease inelectric resistance between two electrodes in a state where a currentdoes not flow. A significant decrease in resistance value (electricresistance) tends to become significant at the time of reading a servopattern written in the magnetic layer of magnetic tape including themagnetic layer having the magnetic layer surface roughness Ra equal toor smaller than 2.0 nm. However, such a significant decrease inresistance value can be prevented by setting the logarithmic decrementto be equal to or smaller than 0.050, in the magnetic tape in which themagnetic layer surface roughness Ra is equal to or smaller than 2.0 nm.

In one preferred aspect, in the magnetic tape device, it is possible toperform the head tracking servo by using the TMR head as the servo headin a case of recording information on the magnetic layer having a servopattern at linear recording density equal to or greater than 250 kfciand/or reproducing information recorded. The unit, kfci, is a unit oflinear recording density (not able to convert to the SI unit system).The linear recording density can be, for example, 250 to 800 kfci andcan also be 300 to 800 kfci. The linear recording density can be, forexample, equal to or smaller than 800 kfci and can also exceed 800 kfci.In the magnetic tape for high-density recording, a width of the servoband tends to decrease, in order to provide a large amount of data bandsin the magnetic layer, and thus, the SNR at the time of reading a servopattern easily decrease. However, a decrease in SNR can be prevented bysetting the magnetic layer surface roughness Ra and the magnetic clusterarea ratio Sdc/Sac of the magnetic tape in the magnetic tape device tobe in the ranges described above.

The servo head is a magnetic head including at least the TMR element asa servo pattern reading element. The servo head may include or may notinclude a reproducing element for reproducing information recorded onthe magnetic tape. That is, the servo head and the reproducing head maybe one magnetic head or separated magnetic heads. The same applies to arecording element for performing the recording of information in themagnetic tape.

As the magnetic tape is transported at a high speed in the magnetic tapedevice, it is possible to shorten the time for recording informationand/or the time for reproducing information. Meanwhile, it is desiredthat the magnetic tape is transported at a low speed at the time ofrecording and reproducing information, in order to prevent adeterioration in recording and reproducing characteristics. From theviewpoint described above, in a case of reading a servo pattern by theservo head in order to perform head tracking servo at the time ofrecording and/or reproducing information, a magnetic tape transportationspeed is preferably equal to or lower than 18 m/sec, more preferablyequal to or lower than 15 m/sec, and even more preferably equal to orlower than 10 m/sec. The magnetic tape transportation speed can be, forexample, equal to or higher than 1 m/sec. The magnetic tapetransportation speed is also referred to as a running speed. In theinvention and the specification, the “magnetic tape transportationspeed” is a relative speed between the magnetic tape transported in themagnetic tape device and the servo head in a case where the servopattern is read by the servo head. The magnetic tape transportationspeed is normally set in a control unit of the magnetic tape device. Asthe magnetic tape transportation speed is low, the time for which thesame portion of the TMR head comes into contact with the magnetic tapeincreases at the time of reading the servo pattern, and accordingly,damage on the TMR head more easily occurs and a decrease in resistancevalue easily occurs. In the magnetic tape device according to one aspectof the invention, such a decrease in resistance value can be preventedby using a magnetic tape having the logarithmic decrement equal to orsmaller than 0.050.

Head Tracking Servo Method

One aspect of the invention relates to a head tracking servo methodincluding: reading a servo pattern of a magnetic layer of a magnetictape by a servo head in a magnetic tape device, in which the servo headis a magnetic head including a tunnel magnetoresistance effect typeelement as a servo pattern reading element, the magnetic tape includes anon-magnetic support, and a magnetic layer including ferromagneticpowder and a binding agent on the non-magnetic support, the magneticlayer includes the servo pattern, a center line average surfaceroughness Ra measured regarding a surface of the magnetic layer is equalto or smaller than 2.0 nm, a logarithmic decrement acquired by apendulum viscoelasticity test performed regarding the surface of themagnetic layer is equal to or smaller than 0.050, and a ratio of anaverage area Sdc of magnetic clusters of the magnetic tape in a DCdemagnetization state and an average area Sac of magnetic clustersthereof in an AC demagnetization state measured with a magnetic forcemicroscope (Sdc/Sac) is 0.80 to 1.30. The reading of the servo patternis performed by bringing the magnetic tape into contact with the servohead allowing sliding while transporting (causing running of) themagnetic tape. The details of the magnetic tape and the servo head usedin the head tracking servo method are as the descriptions regarding themagnetic tape device according to one aspect of the invention.

Hereinafter, as one specific aspect of the head tracking servo, headtracking servo in the timing-based servo system will be described.However, the head tracking servo of the invention is not limited to thefollowing specific aspect.

In the head tracking servo in the timing-based servo system(hereinafter, referred to as a “timing-based servo”), a plurality ofservo patterns having two or more different shapes are formed in amagnetic layer, and a position of a servo head is recognized by aninterval of time in a case where the servo head has read the two servopatterns having different shapes and an interval of time in a case wherethe servo head has read two servo patterns having the same shapes. Theposition of the magnetic head of the magnetic tape in the widthdirection is controlled based on the position of the servo headrecognized as described above. In one aspect, the magnetic head, theposition of which is controlled here, is a magnetic head (reproducinghead) which reproduces information recorded on the magnetic tape, and inanother aspect, the magnetic head is a magnetic head (recording head)which records information in the magnetic tape.

FIG. 5 shows an example of disposition of data bands and servo bands. InFIG. 5, a plurality of servo bands 10 are disposed to be interposedbetween guide bands 12 in a magnetic layer of a magnetic tape 1. Aplurality of regions 11 each of which is interposed between two servobands are data bands. The servo pattern is a magnetized region and isformed by magnetizing a specific region of the magnetic layer by a servowrite head. The region magnetized by the servo write head (positionwhere a servo pattern is formed) is determined by standards. Forexample, in an LTO Ultrium format tape which is based on a localstandard, a plurality of servo patterns tilted in a tape width directionas shown in FIG. 6 are formed on a servo band in a case of manufacturinga magnetic tape. Specifically, in FIG. 6, a servo frame SF on the servoband 10 is configured with a servo sub-frame 1 (SSF1) and a servosub-frame 2 (SSF2). The servo sub-frame 1 is configured with an A burst(in FIG. 6, reference numeral A) and a B burst (in FIG. 6, referencenumeral B). The A burst is configured with servo patterns A1 to A5 andthe B burst is configured with servo patterns B1 to B5. Meanwhile, theservo sub-frame 2 is configured with a C burst (in FIG. 6, referencenumeral C) and a D burst (in FIG. 6, reference numeral D). The C burstis configured with servo patterns C1 to C4 and the D burst is configuredwith servo patterns D1 to D4. Such 18 servo patterns are disposed in thesub-frames in the arrangement of 5, 5, 4, 4, as the sets of 5 servopatterns and 4 servo patterns, and are used for recognizing the servoframes. FIG. 6 shows one servo frame for explaining. However, inpractice, in the magnetic layer of the magnetic tape in which the headtracking servo in the timing-based servo system is performed, aplurality of servo frames are disposed in each servo band in a runningdirection. In FIG. 6, an arrow shows the running direction. For example,a LTO Ultrium format tape generally includes 5,000 or more servo framesper a tape length of 1 m, in each servo band of the magnetic layer. Theservo head sequentially reads the servo patterns in the plurality ofservo frames, while coming into contact with and sliding on the surfaceof the magnetic layer of the magnetic tape transported in the magnetictape device.

In the head tracking servo in the timing-based servo system, a positionof a servo head is recognized based on an interval of time in a casewhere the servo head has read the two servo patterns (reproduced servosignals) having different shapes and an interval of time in a case wherethe servo head has read two servo patterns having the same shapes. Thetime interval is normally obtained as a time interval of a peak of areproduced waveform of a servo signal. For example, in the aspect shownin FIG. 6, the servo pattern of the A burst and the servo pattern of theC burst are servo patterns having the same shapes, and the servo patternof the B burst and the servo pattern of the D burst are servo patternshaving the same shapes. The servo pattern of the A burst and the servopattern of the C burst are servo patterns having the shapes differentfrom the shapes of the servo pattern of the B burst and the servopattern of the D burst. An interval of the time in a case where the twoservo patterns having different shapes are read by the servo head is,for example, an interval between the time in a case where any servopattern of the A burst is read and the time in a case where any servopattern of the B burst is read. An interval of the time in a case wherethe two servo patterns having the same shapes are read by the servo headis, for example, an interval between the time in a case where any servopattern of the A burst is read and the time in a case where any servopattern of the C burst is read. The head tracking servo in thetiming-based servo system is a system supposing that occurrence of adeviation of the time interval is due to a position change of themagnetic tape in the width direction, in a case where the time intervalis deviated from the set value. The set value is a time interval in acase where the magnetic tape runs without occurring the position changein the width direction. In the timing-based servo system, the magnetichead is moved in the width direction in accordance with a degree of thedeviation of the obtained time interval from the set value.Specifically, as the time interval is greatly deviated from the setvalue, the magnetic head is greatly moved in the width direction. Thispoint is applied to not only the aspect shown in FIGS. 5 and 6, but alsoto entire timing-based servo systems.

For the details of the head tracking servo in the timing-based servosystem, well-known technologies such as technologies disclosed in U.S.Pat. No. 5,689,384A, U.S. Pat. No. 6,542,325B, and U.S. Pat. No.7,876,521B can be referred to, for example. In addition, for the detailsof the head tracking servo in the amplitude-based servo system,well-known technologies disclosed in U.S. Pat. No. 5,426,543A and U.S.Pat. No. 5,898,533A can be referred to, for example.

According to one aspect of the invention, a magnetic tape used in amagnetic tape device in which a TMR head is used as a servo head, themagnetic tape including: a magnetic layer including ferromagnetic powderand a binding agent on a non-magnetic support, in which the magneticlayer includes a servo pattern, a center line average surface roughnessRa measured regarding a surface of the magnetic layer is equal to orsmaller than 2.0 nm, a logarithmic decrement acquired by a pendulumviscoelasticity test performed regarding the surface of the magneticlayer is equal to or smaller than 0.050, and a ratio of an average areaSdc of magnetic clusters of the magnetic tape in a DC demagnetizationstate and an average area Sac of magnetic clusters thereof in an ACdemagnetization state measured with a magnetic force microscope(Sdc/Sac) is 0.80 to 1.30, is also provided. The details of the magnetictape are also as the descriptions regarding the magnetic tape deviceaccording to one aspect of the invention.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description mean “parts by mass” and“mass %”, unless otherwise noted. In addition, steps and evaluationsdescribed below are performed in an environment of an atmospheretemperature of 23° C.±1° C., unless otherwise noted.

Example 1

1. Manufacturing of Magnetic Tape

(1) Preparation of Alumina Dispersion

3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo ChemicalIndustry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixedsolvent of methyl ethyl ketone and toluene) of a polyester polyurethaneresin having a SO₃Na group as a polar group (UR-4800 (amount of a polargroup: 80 meq/kg) manufactured by Toyobo Co., Ltd.), and 570.0 parts ofa mixed solution of methyl ethyl ketone and cyclohexanone (mass ratio of1:1) as a solvent were mixed in 100.0 parts of alumina powder (HIT-80manufactured by Sumitomo Chemical Co., Ltd.) having an gelatinizationratio of 65% and a BET specific surface area of 20 m²/g, and dispersedin the presence of zirconia beads by a paint shaker for 5 hours. Afterthe dispersion, the dispersion liquid and the beads were separated by amesh and an alumina dispersion was obtained.

(2) Magnetic Layer Forming Composition List

Magnetic Solution

Ferromagnetic hexagonal barium ferrite powder: 100.0 parts

-   -   Average particle size and saturation magnetization as: see Table        1

Polyurethane resin A: see Table 1

Cyclohexanone: 150.0 parts

Methyl ethyl ketone: 150.0 parts

Abrasive Liquid

Alumina dispersion prepared in the section (1): 6.0 parts

Silica Sol

Colloidal silica: 2.0 parts

-   -   Average particle size: see Table 1

Methyl ethyl ketone: 1.4 parts

Other Components

Stearic acid: 2.0 parts

Stearic acid amide: 0.2 parts

Butyl stearate: 2.0 parts

Polyisocyanate (CORONATE (registered trademark) L manufactured by NipponPolyurethane Industry Co., Ltd.): 2.5 parts

Finishing Additive Solvent

Cyclohexanone: 200.0 parts

Methyl ethyl ketone: 200.0 parts

(3) Non-Magnetic Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

-   -   Average particle size (average long axis length): 0.15 μm    -   Average acicular ratio: 7    -   BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

A vinyl chloride copolymer: 13.0 parts

SO₃Na group-containing polyurethane resin: 9.0 parts

-   -   Weight-average molecular weight: 70,000, SO₃Na group: 0.2 meq/g

Phenylphosphonic acid: 3.0 parts

Stearic acid: 2.0 parts

Stearic acid amide: 0.2 parts

Butyl stearate: 2.0 parts

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

(4) Back Coating Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide: 80.0 parts

-   -   Average particle size (average long axis length): 0.15 μm    -   Average acicular ratio: 7    -   BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

A vinyl chloride copolymer: 13.0 parts

SO₃Na group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Methyl ethyl ketone: 155.0 parts

Stearic acid: 3.0 parts

Butyl stearate: 3.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 355.0 parts

(5) Preparation of Each Layer Forming Composition

The magnetic layer forming composition was prepared by the followingmethod.

The magnetic solution was prepared by dispersing (beads-dispersing) eachcomponent by using a batch type vertical sand mill for 24 hours.Zirconia beads having a bead diameter of 0.1 mm were used as thedispersion beads. The prepared magnetic solution and the abrasive liquidwere mixed with other components (silica sol, other components, andfinishing additive solvent) and beads-dispersed for 5 minutes by usingthe sand mill, and the treatment (ultrasonic dispersion) was performedwith a batch type ultrasonic device (20 kHz, 300 W) for 0.5 minutes.After that, the obtained mixed liquid was filtered by using a filterhaving a hole diameter of 0.5 μm, and the magnetic layer formingcomposition was prepared.

The non-magnetic layer forming composition was prepared by the followingmethod. Each component excluding the lubricant (stearic acid, stearicacid amide, and butyl stearate), cyclohexanone, and methyl ethyl ketonewas dispersed by using a batch type vertical sand mill for 24 hours toobtain dispersion liquid. As the dispersion beads, zirconia beads havinga bead diameter of 0.1 mm were used. After that, the remainingcomponents were added into the obtained dispersion liquid and stirredwith a dissolver. The dispersion liquid obtained as described above wasfiltered with a filter having a hole diameter of 0.5 μm and anon-magnetic layer forming composition was prepared.

The back coating layer forming composition was prepared by the followingmethod. Each component excluding the lubricant (stearic acid and butylstearate), polyisocyanate, and cyclohexanone was kneaded and diluted byan open kneader. Then, the obtained mixed liquid was subjected to adispersing process of 12 passes, with a transverse beads mill dispersingdevice by using zirconia beads having a bead diameter of 1 mm, bysetting a bead filling percentage as 80 volume %, a circumferentialspeed of rotor distal end as 10 m/sec, and a retention time for 1 passas 2 minutes. After that, the remaining components were added into theobtained dispersion liquid and stirred with a dissolver. The dispersionliquid obtained as described above was filtered with a filter having ahole diameter of 1 μm and a back coating layer forming composition wasprepared.

(6) Manufacturing Method of Magnetic Tape

A magnetic tape was manufactured by the specific aspect shown in FIG. 4.The magnetic tape was specifically manufactured as follows.

A support made of polyethylene naphthalate having a thickness of 5.00 μmwas sent from the sending part, and the non-magnetic layer formingcomposition prepared in the section (5) (ii) was applied to one surfacethereof so that the thickness after the drying becomes 1.00 μm in thefirst coating part and was dried in the first heating process zone(atmosphere temperature of 100° C.) to form a coating layer.

Then, the magnetic layer forming composition was applied onto thenon-magnetic layer so that the thickness after the drying becomes 70 nm(0.07 μm) in the second coating part, and a coating layer was formed.The cooling step was performed by passing the formed coating layerthrough the cooling zone in which the atmosphere temperature is adjustedto 0° C. for the staying time shown in Table 1 while the coating layeris wet, and then, the smoothing process was performed with respect tothe coating layer. The smoothing process was performed by applying shearto the coating layer using a commercially available solid smoother(center line average surface roughness (catalogue value): 1.2 nm). Afterthat, a homeotropic alignment process was performed in the orientationzone by applying a magnetic field having a magnetic field strength of0.3 T in a vertical direction with respect to the surface of the coatinglayer, and the coating layer was dried in the second heating processzone (atmosphere temperature of 100° C.).

After that, in the third coating part, the back coating layer formingcomposition was applied to the surface of the support made ofpolyethylene naphthalate on a side opposite to the surface where thenon-magnetic layer and the magnetic layer are formed, so that thethickness after the drying becomes 0.40 μm, to form a coating layer, andthe formed coating layer was dried in the third heating process zone(atmosphere temperature of 100° C.).

The magnetic tape obtained as described above was slit to have a widthof ½ inches (0.0127 meters), and the burnishing treatment and the wipingtreatment were performed with respect to the surface of the coatinglayer of the magnetic layer forming composition. The burnishingtreatment and the wiping treatment were performed by using acommercially available polishing tape (product name: MA22000manufactured by Fujifilm Corporation, abrasive: diamond/Cr₂O₃/red oxide)as the polishing tape, a commercially available sapphire blade(manufactured by Kyocera Corporation, a width of 5 mm, a length of 35mm, and a tip angle of 60 degrees) as the grinding blade, and acommercially available wiping material (product name: WRP736manufactured by Kuraray Co., Ltd.) as the wiping material, in atreatment device having a configuration disclosed in FIG. 1 ofJP1994-52544A (JP-H06-52544A). For the treatment conditions, thetreatment conditions disclosed in Example 12 of JP1994-52544A(JP-H06-52544A).

After the burnishing treatment and the wiping treatment, a calenderprocess (surface smoothing treatment) was performed with a calender rollconfigured of only a metal roll, at a speed of 80 m/min, linear pressureof 300 kg/cm (294 kN/m), and a calender temperature (surface temperatureof a calender roll) shown in Table 1.

After that, a heating process (curing process) was performed in theenvironment of the atmosphere temperature of 70° C. for 36 hours.

By doing so, a magnetic tape for forming a servo pattern on the magneticlayer was manufactured.

In a state where the magnetic layer of the manufactured magnetic tapewas demagnetized, servo patterns having disposition and shapes accordingto the LTO Ultrium format were formed on the magnetic layer by using aservo write head mounted on a servo tester. Accordingly, a magnetic tapeincluding data bands, servo bands, and guide bands in the dispositionaccording to the LTO Ultrium format in the magnetic layer, and includingservo patterns having the disposition and the shape according to the LTOUltrium format on the servo band is manufactured. The servo testerincludes a servo write head and a servo head. This servo tester was alsoused in evaluations which will be described later.

The thickness of each layer of the manufactured magnetic tape isacquired by the following method, and it was confirmed that thethicknesses obtained is the method described above.

A cross section of the magnetic tape in a thickness direction wasexposed to ion beams and the exposed cross section was observed with ascanning electron microscope. Various thicknesses were obtained as anarithmetical mean of thicknesses obtained at two portions in thethickness direction in the cross section observation.

A part of the magnetic tape manufactured by the method described abovewas used in the evaluation described below, and the other part was usedin order to measure an SNR and a resistance value of the TMR head whichwill be described later.

2. Evaluation of Physical Properties of Ferromagnetic Powder

(1) Average Particle Size of Ferromagnetic Powder

An average particle size of the ferromagnetic powder was obtained by themethod described above.

(2) Saturation Magnetization Us of Ferromagnetic Powder

The saturation magnetization as of the ferromagnetic powder was measuredwith an applied magnetic field of 796 kA/m (10 kOe) by using anoscillation sample type magnetic-flux meter (manufactured by ToeiIndustry Co., Ltd.).

The evaluation was performed in Examples and Comparative Examples whichwill be described later in the same manner as described above.

3. Evaluation of Physical Properties of Magnetic Tape

(1) Center Line Average Surface Roughness Ra Measured Regarding Surfaceof Magnetic Layer

The measurement regarding a measurement area of 40 μm×40 μm in thesurface of the magnetic layer of the magnetic tape was performed with anatomic force microscope (AFM, Nanoscope 4 manufactured by VeecoInstruments, Inc.) in a tapping mode, and a center line average surfaceroughness Ra was acquired. RTESP-300 manufactured by BRUKER is used as aprobe, a scan speed (probe movement speed) was set as 40 μm/sec, and aresolution was set as 512 pixel×512 pixel.

(2) Measurement of Logarithmic Decrement

The logarithmic decrement of the surface of the magnetic layer of themagnetic tape was acquired by the method described above by using arigid-body pendulum type physical properties testing instrumentRPT-3000W manufactured by A&D Company, Limited (pendulum: brass,substrate: glass substrate, a rate of temperature increase of substrate:5° C./min) as the measurement device. A measurement sample cut out fromthe magnetic tape was placed on a glass substrate having a size ofapproximately 3 cm×approximately 5 cm, by being fixed at 4 portions witha fixing tape (Kapton tape manufactured by Du Pont-Toray Co., Ltd.) asshown in FIG. 1. An adsorption time was set as 1 second, a measurementinterval was set as 7 to 10 seconds, a displacement-time curve was drawnregarding the 86-th measurement interval, and the logarithmic decrementwas acquired by using this curve. The measurement was performed in theenvironment of relative humidity of approximately 50%.

(3) Magnetic Cluster Area Ratio Sdc/Sac

The Sdc and Sac were obtained by the method described above and themagnetic cluster area ratio Sdc/Sac was calculated from the obtainedvalues. As a magnetic force microscope, Dimension 3100 manufactured byBruker in a frequency modulation mode was used, and as a probe, SSS-MFMR(nominal radius of curvature of 15 nm) manufactured by NanoWorld AG wasused. A distance between the surface of the magnetic layer and a distalend of the probe at the time of the magnetic force microscopeobservation is 20 nm. As image analysis software, MATLAB manufactured byMath Works was used.

4. Measurement of SNR

The servo head of the servo tester was replaced with a commerciallyavailable TMR head (element width of 70 nm) as a reproducing head forHDD. The reading of a servo pattern was performed by attaching themagnetic tape manufactured in the section 1. to the servo tester, andthe SNR was obtained as a ratio of the output and noise. The SNR wascalculated as a relative value by setting the SNR measured as 0 dB inComparative Example 1 which will be described later. In a case where theSNR calculated as described above is equal to or greater than 7.0 dB, itis possible to evaluate that a function of dealing with future needsaccompanied with high-density recording is obtained.

5. Measurement of Resistance Value of Servo Head

The servo head of the servo tester was replaced with a commerciallyavailable TMR head (element width of 70 nm) as a reproducing head forHDD. In the servo tester, the magnetic tape manufactured in the part 1.was transported while bringing the surface of the magnetic layer intocontact with the servo head and causing sliding therebetween. A tapelength of the magnetic tape was 1,000 m, and a total of 4,000 passes ofthe transportation (running) of the magnetic tape was performed bysetting the magnetic tape transportation speed (relative speed of themagnetic tape and the servo head) at the time of the transportation as 4m/sec. The servo head was moved in a width direction of the magnetictape by 2.5 μm for 1 pass, a resistance value (electric resistance) ofthe servo head for transportation of 400 passes was measured, and a rateof a decrease in resistance value with respect to an initial value(resistance value at 0 pass) was obtained by the following equation.Rate of decrease in resistance value (%)=[(initial value−resistancevalue after transportation of 400 passes)/initial value]×100

The measurement of the resistance value (electric resistance) wasperformed by bringing an electric resistance measuring device (digitalmulti-meter (product number: DA-50C) manufactured by Sanwa ElectricInstrument Co., Ltd.) into contact with a wiring connecting twoelectrodes of a TMR element included in a TMR head. In a case where thecalculated rate of a decrease in resistance value was equal to orgreater than 30%, it was determined that a decrease in resistance valueoccurred. Then, a servo head was replaced with a new head, andtransportation after 400 passes was performed and a resistance value wasmeasured. The number of times of occurrence of a decrease in resistancevalue which is 1 or greater indicates a significant decrease inresistance value. In the running of 4,000 passes, in a case where therate of a decrease in resistance value did not become equal to orgreater than 30%, the number of times of occurrence of a decrease inresistance value was set as 0. In a case where the number of times ofoccurrence of a decrease in resistance value is 0, the maximum value ofthe measured rate of a decrease in resistance value is shown in Table 1.

Examples 2 to 9 and Comparative Examples 1 to 10

1. Manufacturing of Magnetic Tape

A magnetic tape was manufactured in the same manner as in Example 1,except that various conditions shown in Table 1 were changed as shown inTable 1.

As the polyurethane resin A shown in Table 1, a polyurethane resin Aused in examples of JP4001532B is used.

As the polyurethane resin B shown in Table 1, a polyurethane resin Bused in comparative examples of JP4001532B is used.

As the vinyl chloride resin shown in Table 1, MR-110 manufactured byZeon Corporation is used.

The polyurethane resin A is a binding agent having higher affinity withthe solvent used in the preparation of the magnetic layer formingcomposition, compared to the polyurethane resin B and the vinyl chlorideresin.

In Table 1, in the comparative examples in which “not performed” isdisclosed in a column of the cooling zone staying time and a column ofthe burnishing treatment before the curing process, a magnetic tape wasmanufactured by a manufacturing step not including a cooling zone in themagnetic layer forming step and not performing the burnishing treatmentand the wiping treatment before the curing process.

In Table 1, in the examples and the comparative examples in which“performed” is disclosed in a column of the smoothing process, thesmoothing process was used in the same manner as in Example 1. In thecomparative examples in which “not performed” is disclosed in a columnof the smoothing process, the magnetic tape was manufactured by amanufacturing step in which the smoothing process is not performed.

2. Evaluation of Physical Properties of Magnetic Tape

Various physical properties of the manufactured magnetic tape wereevaluated in the same manner as in Example 1.

3. Measurement of SNR

The SNR was measured by the same method as that in Example 1, by usingthe manufactured magnetic tape. In Examples 2 to 9 and ComparativeExamples 5 to 10, the TMR head which was the same as that in Example 1was used as a servo head. In Comparative Examples 1 to 4, a commerciallyavailable spin valve type GMR head (element width of 70 nm) was used asa servo head.

4. Measurement of Resistance Value of Servo Head

A resistance value of the servo head was measured by the same method asthat in Example 1, by using the manufactured magnetic tape. As the servohead, the same servo head (TMR head or GMR head) as the servo head usedin the measurement of the SNR was used. In Comparative Examples 1 to 4,the GMR head used as the servo head was a magnetic head having a CIPstructure including two electrodes with an MR element interposedtherebetween in a direction orthogonal to the transportation directionof the magnetic tape. A resistance value was measured in the same manneras in Example 1, by bringing an electric resistance measuring deviceinto contact with a wiring connecting these two electrodes.

The results of the evaluations described above are shown in Table 1.

TABLE 1 Magnetic layer forming Ferromagnetic Type and amount powdercomposition of Colloidal Average binding agent (part) silica Burnishingparticle Poly- Poly- Vinyl average Calendar treatment size σs urethaneurethane chloride Smoothing particle tempera- Cooling zone before (nm (A· m²/kg) resin A resin B resin process size ture staying time curingprocess Comparative 25 50 8.0 2.0 Not Performed 120 nm  80° C. Notperformed Not performed Example 1 Comparative 25 50 8.0 2.0 NotPerformed 120 nm  90° C. Not performed Not performed Example 2Comparative 25 50 8.0 2.0 Not Performed  80 nm  90° C. Not performed Notperformed Example 3 Comparative 25 50 8.0 2.0 Not Performed  40 nm 110°C. Not performed Not performed Example 4 Comparative 25 50 8.0 2.0 NotPerformed 120 nm  80° C. Not performed Not performed Example 5Comparative 25 50 8.0 2.0 Not Performed 120 nm  90° C. Not performed Notperformed Example 6 Comparative 25 50 8.0 2.0 Not Performed  80 nm  90°C. Not performed Not performed Example 7 Comparative 25 50 8.0 2.0 NotPerformed  40 nm 110° C. Not performed Not performed Example 8Comparative 25 50 10.0 Performed  80 nm  90° C. Not performed Notperformed Example 9 Comparative 25 50 8.0 2.0 Not Performed  80 nm  90°C.  1 second Performed Example 10 Example 1 25 50 10.0 Performed  80 nm 90° C.  1 second Performed Example 2 25 50 11.0 Performed  80 nm  90°C.  1 second Performed Example 3 25 50 12.0 Performed  80 nm  90° C.  1second Performed Example 4 25 50 15.0 Performed  80 nm  90° C.  1 secondPerformed Example 5 27 50 10.0 Performed  80 nm  90° C.  1 secondPerformed Example 6 20 50 13.0 Performed  80 nm  90° C.  1 secondPerformed Example 7 25 50 12.0 Performed  80 nm  90° C.  60 secondsPerformed Example 8 25 50 12.0 Performed  80 nm  90° C. 180 secondsPerformed Example 9 25 50 12.0 Performed  40 nm 110° C. 180 secondsPerformed Magnetic Number of layer times of center Logarith- ocurrenceRate of line mic of decrease average decrement decrease in in resis-surface of Magnetic cluster resistance tance roughness magnetic Sdc SacSdc/ Servo SNR value value Ra layer (nm²) (nm²) Sac head (dB) (times)(%) Comparative 2.8 nm 0.06 23000 15000 1.53 GMR 0 0 0 Example 1Comparative 2.5 nm 0.06 23000 15000 1.53 GMR 2.2 0 0 Example 2Comparative 2.0 nm 0.06 23000 15000 1.53 GMR 4.5 0 0 Example 3Comparative 1.5 nm 0.06 23000 15000 1.53 GMR 6.8 0 0 Example 4Comparative 2.8 nm 0.06 23000 15000 1.53 TMR 0.7 1 — Example 5Comparative 2.5 nm 0.06 23000 15000 1.53 TMR 3.2 3 — Example 6Comparative 2.0 nm 0.06 23000 15000 1.53 TMR 5.5 7 — Example 7Comparative 1.5 nm 0.06 23000 15000 1.53 TMR 7.7 9 — Example 8Comparative 2.0 nm 0.06 19000 15000 1.27 TMR 7.0 10 — Example 9Comparative 2.0 nm 0.048 23000 15000 1.53 TMR 5.5 0 5 Example 10 Example1 2.0 nm 0.048 19000 15000 1.27 TMR 7.0 0 5 Example 2 2.0 nm 0.048 1600015000 1.07 TMR 7.2 0 5 Example 3 2.0 nm 0.048 14000 15000 0.93 TMR 7.5 06 Example 4 2.0 nm 0.048 12000 15000 0.80 TMR 7.3 0 7 Example 5 2.0 nm0.048 24000 21000 1.14 TMR 7.2 0 5 Example 6 2.0 nm 0.048 11000 120000.92 TMR 7.5 0 4 Example 7 2.0 nm 0.033 14000 15000 0.93 TMR 7.5 0 2Example 8 2.0 nm 0.015 14000 15000 0.93 TMR 7.5 0 1 Example 9 1.5 nm0.015 14000 15000 0.93 TMR 9.3 0 11

As shown in Table 1, in Examples 1 to 9, the servo pattern could be readat a high SNR by using the TMR head as the servo head. In Examples 1 to9, a significant decrease in resistance value of the TMR head could beprevented.

The invention is effective for usage of magnetic recording for whichhigh-sensitivity reproducing of information recorded with high densityis desired.

What is claimed is:
 1. A magnetic tape device comprising: a magnetictape; and a servo head, wherein the servo head is a magnetic headincluding a tunnel magnetoresistance effect type element as a servopattern reading element, the magnetic tape includes a non-magneticsupport, and a magnetic layer including ferromagnetic powder and abinding agent on the non-magnetic support, the magnetic layer includes aservo pattern, a center line average surface roughness Ra measuredregarding a surface of the magnetic layer is equal to or smaller than2.0 nm, a logarithmic decrement acquired by a pendulum viscoelasticitytest performed regarding the surface of the magnetic layer is equal toor smaller than 0.050, and a ratio Sdc/Sac of an average area Sdc of amagnetic cluster of the magnetic tape in a DC demagnetization state andan average area Sac of a magnetic cluster of the magnetic tape in an ACdemagnetization state measured with a magnetic force microscope is 0.80to 1.30.
 2. The magnetic tape device according to claim 1, wherein thelogarithmic decrement is 0.010 to 0.050.
 3. The magnetic tape deviceaccording to claim 2, wherein the center line average surface roughnessRa measured regarding the surface of the magnetic layer is 1.2 nm to 2.0nm.
 4. The magnetic tape device according to claim 1, wherein the centerline average surface roughness Ra measured regarding the surface of themagnetic layer is 1.2 nm to 2.0 nm.
 5. The magnetic tape deviceaccording to claim 1, wherein the magnetic tape includes a non-magneticlayer including non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.
 6. A head tracking servomethod comprising: reading a servo pattern of a magnetic layer of amagnetic tape by a servo head in a magnetic tape device, wherein theservo head is a magnetic head including a tunnel magnetoresistanceeffect type element as a servo pattern reading element, the magnetictape includes a non-magnetic support, and a magnetic layer includingferromagnetic powder and a binding agent on the non-magnetic support,the magnetic layer includes the servo pattern, a center line averagesurface roughness Ra measured regarding a surface of the magnetic layeris equal to or smaller than 2.0 nm, a logarithmic decrement acquired bya pendulum viscoelasticity test performed regarding the surface of themagnetic layer is equal to or smaller than 0.050, and a ratio Sdc/Sac ofan average area Sdc of a magnetic cluster of the magnetic tape in a DCdemagnetization state and an average area Sac of a magnetic cluster ofthe magnetic tape in an AC demagnetization state measured with amagnetic force microscope is 0.80 to 1.30.
 7. The head tracking servomethod according to claim 6, wherein the logarithmic decrement is 0.010to 0.050.
 8. The head tracking servo method according to claim 7,wherein the center line average surface roughness Ra measured regardingthe surface of the magnetic layer is 1.2 nm to 2.0 nm.
 9. The headtracking servo method according to claim 6, wherein the center lineaverage surface roughness Ra measured regarding the surface of themagnetic layer is 1.2 nm to 2.0 nm.
 10. The head tracking servo methodaccording to claim 6, wherein the magnetic tape includes a non-magneticlayer including non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.